U.S. patent number 8,968,881 [Application Number 11/792,353] was granted by the patent office on 2015-03-03 for precoated metal sheet and method of production of precoated metal sheet.
This patent grant is currently assigned to Nippon Steel & Sumitomo Metal Corporation. The grantee listed for this patent is Hiroyasu Furukawa, Kenji Inada, Hiroshi Kanai, Yoshihiro Suemune, Shigenori Tanaka, Katsunori Tobisawa, Kohei Ueda, Kengo Yoshida. Invention is credited to Hiroyasu Furukawa, Kenji Inada, Hiroshi Kanai, Yoshihiro Suemune, Shigenori Tanaka, Katsunori Tobisawa, Kohei Ueda, Kengo Yoshida.
United States Patent |
8,968,881 |
Furukawa , et al. |
March 3, 2015 |
**Please see images for:
( Certificate of Correction ) ** |
Precoated metal sheet and method of production of precoated metal
sheet
Abstract
A precoated metal sheet produced by simultaneous multilayer
coating, and a process for producing the same, is provided with
improved color shading prevention and adhesion. In the precoated
metal plate, the centerline average roughness Ra in the coating
film interface is not less than 0.3 .mu.m, and the maximum height
from the waviness center line in the interface is not more than 50%
of the layer thickness. Also provided is (i) a top clear coated
metal plate, which comprises a clear layer as an upper layer, has
an interfacial centerline average roughness Ra of 0.3 to 0.7 .mu.m
and has excellent weathering resistance; and (ii) a precoated metal
plate having excellent processability and corrosion resistance,
which contains not less than 30% by mass of a rust preventive
pigment.
Inventors: |
Furukawa; Hiroyasu (Kimitsu,
JP), Kanai; Hiroshi (Kimitsu, JP), Inada;
Kenji (Kimitsu, JP), Tobisawa; Katsunori
(Kimitsu, JP), Tanaka; Shigenori (Tokyo,
JP), Ueda; Kohei (Futtsu, JP), Yoshida;
Kengo (Kimitsu, JP), Suemune; Yoshihiro (Kimitsu,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Furukawa; Hiroyasu
Kanai; Hiroshi
Inada; Kenji
Tobisawa; Katsunori
Tanaka; Shigenori
Ueda; Kohei
Yoshida; Kengo
Suemune; Yoshihiro |
Kimitsu
Kimitsu
Kimitsu
Kimitsu
Tokyo
Futtsu
Kimitsu
Kimitsu |
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
Nippon Steel & Sumitomo Metal
Corporation (Tokyo, JP)
|
Family
ID: |
36578031 |
Appl.
No.: |
11/792,353 |
Filed: |
December 5, 2005 |
PCT
Filed: |
December 05, 2005 |
PCT No.: |
PCT/JP2005/022709 |
371(c)(1),(2),(4) Date: |
June 04, 2007 |
PCT
Pub. No.: |
WO2006/062214 |
PCT
Pub. Date: |
June 15, 2006 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20080003446 A1 |
Jan 3, 2008 |
|
Foreign Application Priority Data
|
|
|
|
|
Dec 8, 2004 [JP] |
|
|
2004-355356 |
Dec 24, 2004 [JP] |
|
|
2004-373521 |
Dec 24, 2004 [JP] |
|
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2004-374073 |
|
Current U.S.
Class: |
428/612; 427/8;
427/216; 428/687 |
Current CPC
Class: |
B05D
7/14 (20130101); B05D 7/5323 (20130101); B05D
7/50 (20130101); Y10T 428/12993 (20150115); B05D
2202/15 (20130101); Y10T 428/12472 (20150115); B05D
2701/00 (20130101) |
Current International
Class: |
B32B
15/18 (20060101); B05D 7/14 (20060101) |
Field of
Search: |
;428/612,687,141,142,457,458,463,425.8 ;427/216,8 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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63-175674 |
|
Jul 1988 |
|
JP |
|
63-268636 |
|
Nov 1988 |
|
JP |
|
01304934 |
|
Dec 1989 |
|
JP |
|
2-105331 |
|
Apr 1990 |
|
JP |
|
02-270546 |
|
Nov 1990 |
|
JP |
|
4-131165 |
|
May 1992 |
|
JP |
|
06-134925 |
|
May 1994 |
|
JP |
|
6-190334 |
|
Jul 1994 |
|
JP |
|
6-190335 |
|
Jul 1994 |
|
JP |
|
6-190336 |
|
Jul 1994 |
|
JP |
|
06190334 |
|
Jul 1994 |
|
JP |
|
7-24401 |
|
Jan 1995 |
|
JP |
|
07-136578 |
|
May 1995 |
|
JP |
|
8-309917 |
|
Nov 1996 |
|
JP |
|
9-122579 |
|
May 1997 |
|
JP |
|
9-122879 |
|
May 1997 |
|
JP |
|
09-286073 |
|
Nov 1997 |
|
JP |
|
10-005693 |
|
Jan 1998 |
|
JP |
|
10008395 |
|
Jan 1998 |
|
JP |
|
11-019583 |
|
Jan 1999 |
|
JP |
|
2003-293167 |
|
Oct 2003 |
|
JP |
|
2003-326639 |
|
Nov 2003 |
|
JP |
|
2004-176113 |
|
Jun 2004 |
|
JP |
|
2004-338254 |
|
Dec 2004 |
|
JP |
|
2004338254 |
|
Dec 2004 |
|
JP |
|
WO 2004/005579 |
|
Jan 2004 |
|
WO |
|
WO 2004076172 |
|
Sep 2004 |
|
WO |
|
Other References
Machine.sub.--English.sub.--Translation.sub.--JP.sub.--06190334.sub.--A;
Suzuki, Atsushi; Preparation of Coated Metal Sheet; Jul. 12, 1994;
JPO; whole document. cited by examiner .
Machine.sub.--English.sub.--Translation.sub.--JP.sub.--2004338254.sub.--A;
Koizumi, Fumitake; Metallized Paper; Dec. 2, 2004; JPO; whole
document. cited by examiner .
Machine English.sub.--Translation.sub.--JP.sub.--10008395.sub.--A;
Kiyoyanagi, Noriko; Vapor Metallized Paper for Label; Jan. 13,
1998; JPO; whole document. cited by examiner .
Korean Office Action dated May 19, 2010 issued in corresponding
Korean Application No. 10-2010-7000334. cited by applicant .
Journal of the Japan Coating Technology Association, 2001, vol. 36,
No. 8, pp. 266 to 271. cited by applicant .
Allowance of Patent dated Aug. 9, 2011 issued in corresponding
Korean Application No. 10-2010-7000334, and English translation
thereof. cited by applicant .
European Search Report in European application No. 05814403.1 dated
Mar. 9, 2011. cited by applicant .
European Search Report dated Oct. 25, 2013, issued in corresponding
European Application No. EP 13182604.2. cited by applicant.
|
Primary Examiner: Austin; Aaron
Assistant Examiner: Khan; Tahseen N
Attorney, Agent or Firm: Kenyon & Kenyon LLP
Claims
The invention claimed is:
1. A precoated metal sheet produced by multilayer simultaneous
coating and superior in workability and corrosion resistance,
comprising a lower coat layer and an upper coat layer thereon which
are formed on a metal sheet by a multilayer simultaneous coating
system, wherein the lower coat layer contains an average 30 wt % or
more of a rust preventing pigment, and the upper coat layer
contains a rust preventing pigment the same as the rust preventing
pigment in the lower coat layer by an average content lower than
the average content of the lower coat layer and by a concentration
gradient gradually decreasing the further from the vicinity of the
interface with the lower coat layer, and the centerline average
roughness Ra of the interface between the lower coat layer and
upper coat layer is 0.3 to 0.7 .mu.m, wherein the rust preventing
pigment is not present at the surface of the upper coat layer.
2. The precoated metal sheet as set forth in claim 1, further
comprising a primer layer under the lower coat layer formed by a
multilayer simultaneous coating system.
3. A precoated metal sheet produced by multilayer simultaneous
coating, comprising a metal sheet with two or more layers of coats
formed on a surface of the metal sheet using a multilayer
simultaneous coating system, said precoated metal sheet
characterized in that a centerline average roughness Ra of an
interface of adjacent layers of coats is 0.3 to 0.7 .mu.m and in
that a maximum height of waviness of the interface from its
centerline observed by a magnification of .times.500 is 50% or less
of the thickness of the layer positioned above the interface
measured from said centerline, the waviness having a pitch of 0.5
to 1 mm.
4. The precoated metal sheet as set forth in claim 3, further
comprising a primer layer under the two or more layers of coats
formed using the multilayer simultaneous coating system.
5. The precoated metal sheet as set forth in claim 3, wherein said
adjacent layers are the uppermost layer and the lower coat layer
adjoining said uppermost layer.
6. A top clear type precoated metal sheet, comprising at least a
metal sheet and two or more layers of coats formed on said metal
sheet using a multilayer simultaneous coating system, said
precoated metal sheet characterized in that said coats include at
least a transparent or semitransparent upper clear layer and a
lower coat layer adjoining said upper clear layer and in that a
centerline average roughness Ra at an interface of said upper clear
layer and said lower coat layer observed by a magnification of
.times.500 is 0.3 to 0.7 .mu.m, the waviness having a pitch of 0.5
to 1 mm.
7. The top clear type precoated metal sheet as set forth in claim
6, wherein one or both of the lower coat layer and the upper clear
layer include an ingredient reacting with both the coating resin of
the lower coat layer and the coating resin of the upper clear
layer.
8. The top clear type precoated metal sheet as set forth in claim
6, further comprising a primer layer under the two or more layers
of coats formed using a multilayer simultaneous coating system.
Description
TECHNICAL FIELD
The present invention relates to a precoated metal sheet used for
home electrical appliances, building materials, automobiles,
furniture, and other applications and a method of production of the
same. More particularly, the present invention relates to a
precoated metal sheet produced by multilayer simultaneous coating,
having an interface of the formed multilayer coats exhibit a
predetermined centerline average roughness Ra, and preventing
mottled-like tone and simultaneously improving the adhesion between
the coats, a top clear type precoated metal sheet superior in
weather resistance, and a precoated metal sheet achieving both
workability and corrosion resistance and methods of production of
the same.
BACKGROUND ART
In the past, metal sheets have often been painted after being
worked in order to improve the rust preventing performance and
appearance. For example, steel sheet is often given a baked on
paint after working. Postcoating metal sheet after working
necessitates a large space for the painting work and is not
necessarily good in terms of the work environment either. Further,
disposal of the waste products generated there is necessary.
Explaining this with reference to the case of steel sheet, in
recent years there has been much activity in use of precoated steel
sheet painted in advance so as to eliminate the painting process
after working and thereby lower the costs. In precoated steel
sheet, the base steel sheet is conversion-treated, then formed with
a primer layer and then formed with one or more coat layers. The
primer layer is sometimes omitted. The back surface is often
painted with a single layer called a "service coat".
In the production of conventional precoated steel sheet having two
or more coat layers in addition to the primer layer, the practice
has been to form a single lower layer paint film on the primer
layer using a curtain coater or roll coater, dry and bake it on,
then again use a curtain coater or roll coater to form a single
upper layer paint coat and dry and bake it on.
As the advantages of the precoated steel sheet produced in the
past, two points may be mentioned: (1) the paintin work is
performed by the manufacturer and there is no painting work at the
user, so the solvent can be disposed of all together and (2) an
entire lot can be painted together, so the quality is stable.
On the other hand, the following may be mentioned as difficulties
with conventional precoated steel sheet: (1) When producing the
sheet by the method of coating and drying single layers at a time,
since the sheets are worked after painting, the coats sometimes
peel off at the time of working. (2) From the viewpoint of
production costs, coating and drying two layers repeatedly was
suitable (painting lines for giving three or more layers were
rare), so if desiring to coat three or more layers of coats so as
to improve the performance of the coats, the sheets had to be
passed through a two-layer coat line two times and therefore the
productivity fell.
If it were possible to apply simultaneous multilayer coating at the
time of production of precoated steel sheet, the following
advantages could be anticipated. First, two or more layers of coats
could be simultaneously painted, so the process could be simplified
compared with the conventional repeated painting of single layers.
Second, three or more layer multilayer coats could also be coated
simultaneously, so a further improvement in efficiency could be
expected. Third, in the past, in "wet-on-wet painting" forming
another paint layer on a not yet dried paint layer and
simultaneously drying and baking them on, the adhesion between the
coats is reportedly improved, so the adhesion between coats can be
expected to be improved over that of the conventional repeated
painting of single layers.
In the past, several proposals have been made for multilayer
simultaneous coating of the surface of steel sheet. For example, JP
6-190335 A describes that if making the surface tension of a lower
layer higher than the surface tension of an upper layer in painted
steel sheet produced by a multilayer simultaneous coater, the
sharpness is improved. The Examples describe that "as the lower
layer paint, a polyester based primer paint was used, while as the
upper layer paint, a high molecular weight polyester based top coat
paint was used. The paints were adjusted in surface tension by
surfactants to make the surface tension of the lower layer paint 35
mN/m and make the surface tension of the upper layer paint 28 mN/m.
These paints were simultaneously coated by a slide hopper type
curtain coater so that the dried thickness of the primer coat
became 5 .mu.m and the top coat became 10 .mu.m. The steel sheet
coated with the paints was then dried and cured by an induction
heating oven, whereupon a high sharpness steel sheet could be
obtained."
JP 6-190336 A proposes a method of production of a patterned steel
sheet. The Examples describe use of high molecular weight polyester
based top coat paints as paints, making the paint forming the upper
layer on the surface of the steel sheet white, making the paint
forming the lower layer black, adjusting the surface tensions of
the paints by surfactants so as to create patterns and making the
surface tension of the upper layer paint 35 mN/m and the surface
tension of the lower layer paint 28 mN/m, and simultaneously
coating these two paints by a slide hopper type coater to give
dried thicknesses of 10 .mu.m each.
Production of a precoated metal sheet by multilayer simultaneous
coating using a slide hopper type coater is described in for
example JP 6-190334 A etc.
One type of precoated metal sheet, that is, top clear type
precoated steel sheet, combines a topmost layer clear paint and a
lower layer colored base coat paint so as to give a heavy feel and
luxurious feel to the appearance. This has been used for
automobiles, home electrical appliances (in particular outdoor
electrical appliances), building materials (in particular outdoor
building materials), etc. In general, in such top clear type
precoated steel sheet, as the topmost layer acrylic paint, a
polyester-based, acryl-based, PET-based, silicone polyester-based,
silicone-based, silicone acryl-based, fluorine-based, or other
resin has been used.
In the case of outdoor use, mainly a polyester-based, acryl-based,
fluorine-based, silicone-based, silicone polyester-based, or
silicone acryl-based clear paint has been used. However, a
polyester-based one is inferior in weather resistance, so has the
problem that with long term use, the clear paint deteriorates and
peels off, while an acryl-based one is superior to a
polyester-based one in weather resistance, but is still
insufficient in weather resistance. On the other hand, as an
example of use of a fluorine-based one, for example, JP 4-131165 A
reports use of a water-based fluorine resin for a top clear paint.
JP 10-5693 A reports the method of use of a fluorine resin for a
top clear paint of a cosmetic metal sheet having an appearance
close to that of natural stone or rock. While it is true that use
of a fluorine-based resin for a clear paint in this way improves
the weather resistance, there was the problem that the cost became
higher.
In the case of a top clear type precoated metal sheet of a type
comprised of a plurality of coat layers in this way in which the
topmost layer is a transparent or semitransparent clear layer,
there is the feature that even if the topmost layer deteriorates,
there will be little chalking due to exposure of the pigment and
therefore the appearance will remain superior at the time of long
term use.
However, there was the problem that the interface between the clear
layer and the lower layer coat below it deteriorated due to light
and therefore the lower layer coats and clear layer easily peeled
apart. The reason is believed to be that the major cause of the
deterioration, UV rays, pass through the clear layer and cause the
surfacemost layer of the lower layer coats to deteriorate. To
prevent this deterioration, the methods have been adopted of adding
an expensive UV absorbent to the clear layer (Journal of the Japan
Coating Technology Association, 2001, vol. 36, no. 8, pp. 266 to
271) etc., but in general this is costly and is insufficient in
effect of preventing deterioration.
Precoated metal sheet is shaped at the demand side, so superior
workability is considered of the primary importance, but in
addition to this, in particular corrosion resistance is sought in
some cases. For example, JP 8-309917 A discloses precoated metal
sheet superior in not only workability, but also end face corrosion
resistance. In the precoated metal sheet described in JP 8-309917
A, the two sides of galvanized metal sheet are formed with coats
containing a rust preventing pigment in an amount of 25% or more.
It describes that if the content of the rust preventing pigment
becomes 50%, the workability sometimes falls. Further, it describes
that a topcoat can be formed. The Examples describe an example of
production by coating a paint containing a rust preventing pigment
by a roll coater, then baking it, followed by coating a topcoat
paint by a roll coater again and then baking it, that is, two
coatings and two bakings (in general, abbreviated as the "2 coat, 2
bake (2C2B)").
An ordinary precoated metal sheet is produced by separately forming
a lower layer coat and an upper layer coat by the above explained
method of two coatings and two bakings. The method of forming two
layers of paint film (comprised of a colored base paint layer and a
clear paint layer) by the wet-on-wet method (that is, forming a
clear paint layer over a colored base paint layer before baking)
and baking them once to form two layers of coats (in general,
abbreviated as the "2 coat, 1 bake (2C1B)") is also known (for
example, JP 11-19583 A).
As described in JP 8-309917 A, it is known that if increasing the
content of the rust preventing pigment in the coats, the precoated
metal sheet is impaired in workability. Further, in a general
precoated metal sheet of a two-layer structure of a lower layer and
upper layer, it is known that it is no longer possible to raise the
corrosion resistance of the precoated metal sheet as desired as
will be understood from the fact that if increasing the rust
preventing pigment of the lower layer, saltwater invades the
interface of the two layers in a saltwater spray test (SST) and
causes blisters.
DISCLOSURE OF THE INVENTION
The inventors produced precoated steel sheet using the multilayer
simultaneous coating method described in JP 6-190335 A and JP
6-190336 A whereupon they found that the surfaces of the steel
sheets after painting experience mottled-like tone. "Mottled-like
tone" is the phenomenon where if examining the appearance of the
coats of a coated steel sheet by the naked eye or a loupe,
unevenness comprised of patches of color different from the
surrounding color are recognized.
Further, the inventors expected an improvement on the adhesion
between coats, but learned that the adhesion is not necessarily
always improved.
The present invention has as its object to prevent mottled-like
tone in the surface appearance and simultaneously improve the
adhesion between coats of precoated metal sheet such as precoated
steel sheet produced by multilayer simultaneous coating using a
multilayer simultaneous coater.
The present invention further has as its object the provision of a
top clear type precoated metal sheet solving the above defects of
the prior art and superior in weather resistance.
Provision of a precoated metal sheet achieving both workability and
corrosion resistance is also an object of the present
invention.
The inventors engaged in various experiments to make use of the
characteristics inherent in precoated steel sheet produced by
multilayer simultaneous coating and as a result found, as features
of the interface between multilayer coats obtained by the
multilayer simultaneous coating method, that waviness is recognized
at the interface enlarged by a low magnification of .times.500
(compared with the interface seen when forming the upper and lower
coats by separately coating and baking these paints, an interface
where the upper layer enters the lower layer and the lower layer
enters the upper layer resulting in a wavy shape) and that if
further observing the interface by a high magnification of
.times.5000, fine roughness is seen at the wavy interface. Further,
they discovered that such a wavy interface and fine roughness at
the interface itself can be controlled by adjusting the surface
tensions of the upper layer and lower layer paints and were
therefore able to propose means for simultaneously solving the
problems of adhesion between coats and mottled-like tone of the
coat appearance. Due to this, they realized precoated steel sheet
by multilayer simultaneous coating having properties superior to
those of conventional precoated steel sheet on which single
individually formed layers are laminated. These developed
techniques are not limited to precoated steel sheet and can also be
applied to general precoated metal sheet obtained by forming coats
on unworked metal sheet by multilayer simultaneous coating.
In the present invention, "multilayer simultaneous coating" means
to simultaneously coat a metal sheet with two or more layers of
different paints to form multiple layers of paint films and then
simultaneously dry and bake these paint layers to form multilayer
coats.
Here, the framework of the precoated metal sheet of the present
invention preventing mottled-like tone and simultaneously improving
the adhesion between coats and the methods of production of the
same will be explained below.
(1) A precoated metal sheet produced by multilayer simultaneous
coating comprised of a metal sheet formed on the surface with two
or more layers of coats using a multilayer simultaneous coating
system, said precoated metal sheet characterized in that a
centerline average roughness Ra of the interface of the coats is
0.3 .mu.m or more and in that a maximum height of waviness of the
interface from its centerline observed by a magnification of
.times.500 is 50% or less of the thickness of the layer positioned
above the interface measured from said centerline.
(2) A method of production of precoated metal sheet comprising
applying paints to the surface of a metal sheet using a multilayer
simultaneous coating system, said method of production of precoated
metal sheet characterized by adding a leveler to an upper layer
paint to adjust a difference of surface tensions of a lower layer
paint and upper layer paint to 1.2 mN/m to less than 5 mN/m and
using, as the different layer paints, paints with a difference
between the maximum value and minimum value of measured surface
tensions obtained when measuring the surface tensions five times of
less than 2 mN/m.
(3) A method of production of precoated metal sheet comprising
applying paints to the surface of a metal sheet using a multilayer
simultaneous coating system, said method of production of precoated
metal sheet characterized by adding a leveler to both an upper
layer paint and a lower layer paint to adjust a difference of
surface tensions of the lower layer paint and the upper layer paint
to 0.3 mN/m to less than 3.7 mN/m and using as the different layer
paints paints with a difference between the maximum value and
minimum value of measured surface tensions obtained when measuring
the surface tensions five times of less than 2 mN/m.
In a precoated metal sheet preventing mottled-like tone of the
present invention, an "upper layer" or an "upper coat layer" means
a coat layer positioned at the uppermost position among the colored
layers in the two or more paint layers painted on the surface of a
metal sheet by a multilayer simultaneous coating system, while a
"lower layer" or a "lower coat layer" means a coat layer positioned
adjoining that colored uppermost layer under it. In the case of
forming three or more layers by multilayer simultaneous coating,
when the relationship shown in the above (2) or (3) stands for all
combinations of the adjoining layers simultaneously coated, it is
possible to suppress occurrence of mottled-like tone.
In the present invention, the "leveler" is a type of surface
adjuster of a paint and is also called a "leveling agent". It is an
additive having characteristics leveling the surface of a paint.
Regarding the addition of a suitable leveler, as explained later,
it is possible to judge whether "orange peel like" surface
roughness will occur in the wet state when coating a paint to which
a leveler has been added by a single layer.
According to the present invention, by producing a precoated metal
sheet by multilayer simultaneous coating without allowing the
occurrence of mottled-like tone of the coats, it is possible to
simultaneously coat two or more layers of coats, so it is possible
to simplify the process compared with the conventional repeated
coating of single layers. In the present invention, it is possible
to form multilayer coats of three or more layers by simultaneous
coating and thereby further improve the efficiency.
Further, according to the present invention, it is possible to
provide a precoated metal sheet not inferior in mottled-like tone
and better in adhesion compared with a conventional multilayer
precoated metal sheet produced by repeated coating of single
layers.
The inventors discovered that if controlling the fine roughness at
the wavy interface between coats formed by multilayer simultaneous
coating, it is possible to obtain a top clear type precoated metal
sheet superior in weather resistance.
The top clear type precoated metal sheet superior in weather
resistance of the present invention is based on this discovery.
More particularly, it is a precoated metal sheet including at least
a metal sheet and two or more layers of coats formed on that metal
sheet using a multilayer simultaneous coating system, characterized
in that the coats include at least a transparent or semitransparent
upper clear layer and a lower coat layer adjoining that upper clear
layer and in that a centerline average roughness Ra of an interface
between the upper clear layer and lower coat layer is 0.3 to 0.7
.mu.m.
Preferably, one or both of the lower coat layer and upper clear
layer includes an ingredient (curing agent) reacting with both the
paint resin of that layer and the paint resin of another layer.
In the precoated metal sheet of the present invention having the
above configuration, it is believed that based on the fact that the
Ra of the interface between the uppermost layer (clear layer) and
the lower coat layer adjoining it is 0.3 to 0.7 .mu.m, by the
combination of the increase of the area of the interface and the
increase of the ratio of the irregular reflection at the interface,
it is possible to suppress photo deterioration of the interface
between the clear layer and lower layer coats and secure adhesion
and therefore improve the weather resistance.
As opposed to this, it is believed that, in a conventional
precoated metal sheet having a top clear layer, the light passing
through the top clear layer caused photo deterioration at the
interface with the colored layer at the lower layer adjoining it,
the adhesion was impaired at the interface between the top clear
layer and the colored layer, and peeling easily occurred, so the
precoated metal sheet was insufficient in weather resistance.
As explained above, according to the present invention, it is
possible to obtain a precoated metal sheet where interlayer peeling
between the top clear layer and lower layer coat is effectively
suppressed and the weather resistance is improved.
Further, in the precoated metal sheet of the present invention,
since it is no longer absolutely necessary to use an ultraviolet
(UV) absorbent in the top clear layer (this was essential in a
conventional top clear type coated sheet), it is possible to reduce
or eliminate the amount of the expensive UV absorbent used.
The inventors further discovered that by suppressing the fine
roughness at the wavy interface between the multilayer coats formed
by multilayer simultaneous coating and controlling the contents of
the rust preventing pigment in the coats, it is possible to obtain
a precoated metal sheet achieving both workability and corrosion
resistance.
More specifically, the inventors discovered that by forming by
multilayer simultaneous coating a lower layer containing an average
30 wt % or more of a rust preventing pigment and an upper layer
containing the same rust preventing pigment as the rust preventing
pigment in the lower layer by an average content lower than the
average content of the lower layer and by a concentration gradient
gradually decreasing the further from the vicinity of the interface
with the lower layer and suitably adjusting the centerline average
roughness Ra at the wavy interface of the two layers, a precoated
metal sheet superior in workability and superior in corrosion
resistance can be obtained.
The precoated metal sheet of the present invention achieving both
workability and corrosion resistance is based on this discovery.
Its framework is as follows:
(1) A precoated metal sheet produced by multilayer simultaneous
coating and superior in workability and corrosion resistance
characterized by comprising a lower coat layer and an upper coat
layer thereon which are formed on a metal sheet by a multilayer
simultaneous coating system, wherein the lower coat layer contains
an average 30 wt % or more of a rust preventing pigment, the upper
coat layer contains a rust preventing pigment the same as the rust
preventing pigment in the lower coat layer by an average content
lower than the average content of the lower coat layer and by a
concentration gradient gradually decreasing the further from the
vicinity of the interface with the lower coat layer, and the
centerline average roughness Ra of the interface between the lower
coat layer and upper coat layer is 0.3 to 0.7 .mu.m.
(2) A method of production of precoated metal sheet produced by
multilayer simultaneous coating system and superior in workability
and corrosion resistance, said method of production of precoated
metal sheet characterized by causing part of the rust preventing
pigment in a lower coat layer to disperse in an upper coat layer
when baking a paint film comprising a lower coat layer containing a
rust preventing pigment and an upper coat layer thereon formed by
multilayer simultaneous coating so that the average concentration
of the rust preventing pigment in the lower layer formed by baking
becomes 30 wt % or more and so that the same rust preventing
pigment as the rust preventing pigment in the lower layer is
present in the upper layer by an average content lower than the
average content of the lower layer and by a concentration gradient
gradually decreasing the further from the vicinity of the interface
with the lower layer and by adding a leveler to the upper coat
layer or the upper coat layer and lower coat layer so that the
centerline average roughness Ra of the interface between the lower
coat layer and upper coat layer becomes 0.3 to 0.7 .mu.m.
In the precoated metal sheet produced by multilayer simultaneous
coating and superior in workability and corrosion resistance of the
present invention, it is possible to increase the rust preventing
pigment of the lower layer, so is thereby possible to secure
corrosion resistance and secure adhesion between the upper layer
and lower layer and thereby improve the workability of the
precoated metal sheet and further improve the corrosion
resistance.
The precoated metal sheet achieving both workability and corrosion
resistance of the present invention may be a precoated metal sheet
obtained by simultaneously coating three or more layers or may be a
precoated metal sheet comprising for example two layers including
rust preventing pigments so as to satisfy the requirements of the
present invention and a clear coat serving as a top coat over the
same.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of the relationship between the difference in
surface tensions of a lower layer and an upper layer and a
mottled-like tone score.
FIG. 2 is another view of the relationship between the difference
in surface tensions of a lower layer and an upper layer and a
mottled-like tone score.
FIGS. 3A and 3B are views for explaining the interface of a lower
coat layer and an upper coat layer of a precoated metal sheet
according to the present invention.
FIGS. 4A and 4B are scan type micrographs of the cross-section of
coats of a precoated metal sheet according to the present
invention.
FIG. 5 is a view for explaining a method of evaluation of the Ra of
a coat interface of a lower layer and upper layer.
FIG. 6 is a view for explaining an interface in the case where the
lower layer includes a pigment.
FIG. 7 is a schematic perspective view of a slide hopper type
curtain coating system.
FIG. 8 is a view for explaining a sample of a coated metal sheet
subjected to a saltwater spray test (SST).
FIG. 9 is a view for explaining a cross-cut part of a sample of a
coated metal sheet subjected to a saltwater spray test.
FIG. 10 is a view for explaining an end face of a sample of a
coated metal sheet subjected to a saltwater spray test.
FIG. 11 is a schematic view explaining a production processing line
of a precoated steel sheet used in the examples.
BEST MODE FOR CARRYING OUT THE INVENTION
The precoated metal sheet of the present invention is comprised of
various types of metal sheet having multilayer structure coats of
two or more layers formed by multilayer simultaneous coating. As
the base metal sheet, for example, steel sheet, aluminum sheet,
titanium sheet, etc. may be used, but the metal sheet is not
limited to these.
The metal sheet may also be treated before coating. Further, it may
be coated with a primer paint before the coating by the multilayer
simultaneous coating system both when treating the metal sheet
before coating and when not.
As examples of usable steel sheet, cold rolled steel sheet, hot
rolled steel sheet, galvanized steel sheet, alloyed galvanized
steel sheet, zinc-iron alloy plated steel sheet, zinc-aluminum
alloy plated steel sheet, aluminum plated steel sheet, chrome
plated steel sheet, nickel plated steel sheet, zinc-nickel alloy
plated steel sheet, tin plated steel sheet, or other steel sheet
may be mentioned. The steel sheet may if necessary be pretreated
(treated before coating). As the pretreatment, there are cold water
washing, hot water washing, pickling, alkali degreasing, grinding,
polishing, chromate treatment, zinc phosphate treatment, compound
oxide film treatment, and other nonchromate type treatment etc.
These may be used alone or in combination to treat the steel sheet
before coating.
Below, the present invention will be explained with reference to
the example of a representative metal sheet as constituted by steel
sheet.
In the past, when using a multilayer simultaneous coating system
for multilayer simultaneous coating and simultaneous baking of
steel sheet, as exampled above, the coats sometimes suffered from
"mottled-like tone".
There was no knowledge in the past on what circumstances this
mottled-like tone occurred under. Therefore, first, the inventors
analyzed the occurrence of mottled-like tone and studied the
locations of occurrence of mottled-like tone. For this purpose,
they compared the state of occurrence of mottled-like tone for
multilayer precoated steel sheet dried without passing through a
furnace and multilayer precoated steel sheet after passing through
a furnace. As a result, they found that mottled-like tone did not
occur if drying without passing through a furnace. On the other
hand, they used a furnace for a heating test and observed the sheet
in that case, whereupon they found that when heating by the
ordinary heating rate for precoated steel sheet of 3 to 7.degree.
C./sec, mottled-like tone occurred at some precoated steel sheet at
a temperature of 90 to 120.degree. C. Therefore, they learned that
mottled-like tone occurs in the furnace.
Next, they examined the cross-section of the coats where
mottled-like tone occurred at a magnification of .times.500,
whereupon they observed thick parts and thin parts in the lower
layer coat and, corresponding to this, thin parts and thick parts
of the upper layer coat. Therefore, the interface of the upper and
lower coat layers had the upper layer entering the lower layer and
the lower layer entering the upper layer resulting in a wavy
appearance. However, no mixture of the coats of the upper layer and
lower layer was observed. The interface between the upper layer and
lower layer when forming them separately appears flat when observed
by the same magnification, so it was learned that this wavy
interface was a feature of the interface of coats formed by
multilayer simultaneous coating. From this, it was learned that if
the thicknesses of the upper layer and the lower layer become
excessively uneven, mottled-like tone will be observed when
observing the coated surface of the steel sheet.
Thus, the "mottled-like tone" in the invention is a defect in coat
layers resulting from the waviness due to the mutually penetrating
upper and lower layers, which is observed in a cross section of
stacked coat layers formed by multilayer simultaneous coating. In
coat layers with "mottled-like tone", spots of color different from
the surrounding color are observed when examining the surface of
the stacked coat layers by the naked eye or a loupe. In other
words, the "mottled-like tone" in the invention is a defect in coat
layers, which is observed as spots of color different from the
surrounding color when viewing the coat layer surface, which
results from the portion of the lower layer penetrating into the
upper layer approaching the surface of the upper layer, which leads
to the relatively reduced thickness of the upper layer.
Further, the inventors learned that if the thickness of the coats
as a whole becomes greater, mottled-like tone will easily occur.
Specifically, they learned that if producing precoated steel sheet
with a thickness of the coats after drying of 15 .mu.m or more by
multilayer simultaneous coating, mottled-like tone easily
occurs.
The inventors studied whether these phenomena can be explained by
the knowledge regarding conventional single layer coats.
With single layer coats, for example, a mottling phenomenon called
"floating" has been reported. The reason why this floating occurs
is that the evaporation of the solvent from the paint film after
coating causes the surface layer to increase in viscosity and the
temperature to drop, so the density and surface tension of the
surface substance increase, the surface substance sink into the
paint film, and the parts having a higher solvent content and lower
surface tension rise from the inside of the paint film and spread
at the surface to cause convection. This phenomenon is in general
known as "Benard cell convection".
Therefore, the mottled-like tone phenomenon at the simultaneous
multilayer coating differs from the floating phenomenon at a
conventional single layer, but is a phenomenon occurring in the
furnace, so there is a possibility of the convection in the coating
film having an effect. Considering this, the inventors began their
studies from the method of suppressing the convection.
As explained above, the phenomenon known as Benard cell convection
at a single layer is a convection phenomenon where when the solvent
unevenly volatilizes from the surface of the paint film, the
surface tension or viscosity at that part increases, whereby a
downward flow easily occurs at that part. This phenomenon is also
known as Marangoni convection. It is known that Benard cell
convection can be suppressed by adding a leveler, one type of
surface adjusting agent, to the paint to suppress uneven
evaporation of the solvent from the paint film.
Therefore, the inventors added a leveler to the upper layer paint
among the paints for multilayer simultaneous coating so as to
fabricate a precoated steel sheet and compared the extent of
occurrence of mottled-like tone with a sheet in which a leveler is
not added to the upper layer paint. As a result, they learned that
by adding a leveler to the upper layer, the extent of mottled-like
tone is improved compared with when not adding a leveler to the
upper layer, but the state of occurrence becomes diverse.
Next, the inventors studied whether it is possible to adjust the
surface tensions of the upper layer and lower layer paints under
conditions of adding a leveler to the upper layer paint so as to
prevent mottled-like tone. As a result, they discovered that if
adjusting the difference of the surface tensions of the upper layer
and lower layer paints to a certain range, it is possible to
prevent mottled-like tone.
However, the fact that the difference of surface tensions can be
used to prevent convection in the multilayer coating film cannot be
explained from the mechanism of occurrence of Benard cell
convection due to conventional Marangoni convection. Further, the
phenomenon of Marangoni convection in the case of coating two or
more different layers of paints was not known in the past.
Therefore, they considered the following hypothesis. For an upper
layer paint to spread over a lower layer paint, from the balance of
the forces of the interface, it is believed that the following
equation must stand. S=.sigma.(lower layer)-.sigma.(upper
layer)-.gamma.(between upper layer and lower layer)>0
where, .sigma. is the surface tension, and .gamma. is the
interfacial tension
Therefore, it is believed that the relationship .sigma.(lower
layer)-.sigma.(upper layer)>.gamma.(between upper layer and
lower layer) is necessary. In other words, it is believed that if
the surface tension of the lower layer is not larger than the
surface tension of the upper layer by the amount of the interfacial
tension, the upper layer paint will not spread over the lower layer
paint. It is believed that the force by which the upper layer paint
spreads wet works in a direction suppressing the force due to
Marangoni convection disturbing the interface between the upper
layer and lower layer and broadening the interface.
It is believed that the effect of suppressing mottled-like tone by
adding a leveler as a surface adjusting agent to the upper layer of
the multilayer paint film is due to uniformly reducing the surface
tension of the upper layer and suppressing uneven evaporation of
the solvent. As a result, this is believed to be similar to the
effect of suppression of Benard cell convection at a conventional
single layer paint film.
Further, as a new effect, it is believed that uniformly reducing
the surface tension of the upper layer paint promotes the uniform
spread of the upper layer paint on the lower layer paint, so acts
in a direction uniformly suppressing the force disturbing the
interface between the upper layer and lower layer and broadening
the interface.
The inventors discovered that if introducing a leveler into not
only the upper layer, but also the lower layer, when the surface
tension of the upper layer is lower than the surface tension of the
lower layer, there are conditions under which mottled-like tone
will not occur even when the difference between the surface tension
of the upper layer and the surface tension of the lower layer is
small. This phenomenon is guessed to be that the lower layer paint
easily spreads on the underlying layer (other paint layer formed
together by multilayer simultaneous coating, steel sheet base, or
primer layer on the same) and the phenomenon of the lower layer
paint locally building up is suppressed.
In this way, in the present invention, it is important to adjust
the surface tensions of the upper layer and lower layer paints. The
surface tensions can be adjusted by utilizing a leveler, one type
of surfactant.
In general, surfactants include levelers and defoamers. In
particular, defoamers are being used to remove bubbles from
coatings.
What is being used in the present invention to adjust the surface
tensions is surfactants of the type called "levelers" used for
improving the flatness of coats. Suitable levelers selected from
among these by the following judgment are used. The suitable
levelers and the amounts added are determined simply by whether
orange peel like cissing occurs in the wet state when applying a
paint to which an amount of leveler, required for adjustment of the
surface tension of the paint has been added, in a single layer on a
sufficiently wet sheet (a sheet where the paint layer does not
become an orange peel like state-in the wet state when applying a
paint to which no leveler has been added). A leveler not causing
orange peel like cissing may be judged as a suitable leveler able
to be used in the present invention.
As levelers actually able to be used, there are acryl-based
levelers and silicone-based levelers. For example, as
non-silicone-based types, Homogenol L18, Homogenol L95, and
Homogenol L1820 (Kao Corporation), BYK057, BYK051, BYK052, BYK053,
BYK055, BYK077 (BYK-Chemie), etc. and as silicone-based types,
Homogenol L100 (Kao Corporation), BYK080, BYK141, BYK065, BYK066,
BYK070, and BYK088 (BYK-Chemie), etc. may be mentioned.
The leveler in the coats of the precoated steel sheet produced by
multilayer simultaneous coating can be detected by peeling off the
coats of the product, extracting the unreacted polymer in a
solvent, heating the solvent after extraction to concentrate the
result, then analyzing the result by infrared spectrophotometric
analysis.
Next, the results of study of the prevention of mottled-like tone
of actual experiments will be explained with reference to the
drawings. The mottled-like tone score was expressed using no
mottled-like tone able to be seen as a score of 5 and mottled-like
tone occurring to an extent able to be sufficiently confirmed
visually as a score of 1 and dividing the interval between them
into different extents of occurrence of mottled-like tone. For
example, a score of 4 indicates mottled-like tone which cannot be
clearly seen visually, but is of an extent which can be confirmed
by viewing by a .times.10 loupe.
FIG. 1 is a view of the effect on mottled-like tone when a leveler
is not introduced to the lower layer coat and a leveler is
introduced and not introduced into the upper layer coat. This shows
the relationship between the difference of surface tensions (lower
layer-upper layer) on the abscissa and the mottled-like tone score
on the ordinate.
When the upper layer contains a leveler (shown by the black circles
in FIG. 1), if the difference in surface tensions (lower
layer-upper layer) becomes 1.2 mN/m or more, a mottled-like tone
score of 5 was obtained. On the other hand, when the upper layer
does not include a leveler (shown by triangles in FIG. 1), the
difference in surface tensions between the lower layer and the
upper layer can only be adjusted to about 1 mN/m and the
mottled-like tone score varied from 2 to 4.
When the upper layer does not contain a leveler, the amount of
adjustment of the difference in surface tensions between the lower
layer and the upper layer is small, but compared with the case
where the upper layer contains a leveler in this range, the case
where the upper layer contains a leveler is relatively better in
mottled-like tone score. The addition of a leveler to the upper
layer improves the mottled-like tone.
FIG. 2 is a view of the effect on mottled-like tone when adding a
leveler to the upper layer and adding and not adding a leveler to
the lower layer. It shows the relationship between the two with the
difference in surface tensions (lower layer-upper layer) on the
abscissa and the mottled-like tone score on the ordinate.
When the upper layer and lower layer contain levelers (shown by
white circles in FIG. 2), if the difference in surface tensions
(lower layer-upper layer) becomes 0.3 mN/m or more, a mottled-like
tone score of 5 is obtained. In this case, compared with the case
where the upper layer contains a leveler and the lower layer does
not include a leveler (shown by the black circles in FIG. 2), even
if the difference in surface tensions is small, the mottled-like
tone is improved.
Among the examples of FIGS. 1 and 2 where the upper layer includes
a leveler but the lower layer does not include a leveler, there is
an example where mottled-like tone of a score of 2 is seen when the
difference of surface tensions is 1.1 mN/m. In this example, the
amount of solvent added to the upper layer was increased and the
surface tension of the upper layer was reduced, so the upper layer
also fell in viscosity. Therefore, the first reason for the
occurrence of mottled-like tone was mentioned to be the difference
in surface tensions of the lower layer and upper layer being less
than 1.2 mN/m, but as another reason, the effect of the drop in
viscosity on the mottled-like tone may be considered. The viscosity
of the upper layer measured by an Iwata cup in this example was 37
seconds. However, in another example, even though the difference in
surface tensions of the lower layer and upper layer was less than
1.2 mN/m, when the viscosity of the upper layer was 59 seconds, the
mottled-like tone score was 4. From this, it can be said that the
higher the viscosity of the upper layer, the more the mottled-like
tone is suppressed and that 60 seconds or more is desirable.
Further, the tendency is seen for the viscosity of the lower layer
as well that the higher the viscosity, the more mottled-like tone
is suppressed.
As the paint of each layer used for the multilayer simultaneous
coating in the present invention, it is important to use a paint
giving a difference between the maximum value and minimum value of
surface tension of less than 2 mN/m when measuring the surface
tension five times. This is for the following reason. If micro
unevenness occurs in the surface tension in a coating, this
triggers the above-mentioned fine convection phenomenon in the
coating layer in the heating process. Even if the average
difference of the surface tensions of the upper and lower layers is
suitable, mottled-like tone ends up occurring. Therefore, each
coating used has to be a coating securing micro evenness of the
surface tension.
A paint layer formed by a paint giving rise to micro unevenness of
surface tension exhibits an orange peel like appearance as a result
of the induced fine convection phenomenon. Whether such an orange
peel appearance is exhibited can be simply judged as explained
above by the appearance when applying a paint in a single layer to
a sheet (micro uneven paint ending up with an orange peel
appearance). The inventors discovered that as a standard for
quantitatively judging this, the variation in values when measuring
the surface tension several times is useful. That is, they learned
that when measuring the surface tension five times, if the
difference between the maximum value and the minimum value is less
than 2 mN/m, the paint can be deemed sufficiently microscopically
even.
If examining the cross-section of the precoated steel sheet of the
present invention produced by multilayer simultaneous coating under
.times.500, as shown schematically in FIG. 3A, waviness is observed
in the interface 105 of the lower layer coat layer 101 and upper
layer coat layer 103. In general, the pitch of the waviness
(distance P in the drawing) is 0.5 to 1 mm or so. If examining the
coat layer interface 105 by a further higher magnification of
.times.5000, fine roughness is observed at the interface as shown
schematically in FIG. 3B corresponding to an enlarged view of the
part shown by B in FIG. 3A.
FIG. 4A and FIG. 4B are .times.500 and .times.5000 scan-type
micrographs of cross-sections perpendicular to the surface of the
coats obtained by cutting the coated steel sheet, embedding it in a
resin, then smoothing it by polishing. The bottommost layer in the
three layers seen at the center of FIG. 4A is the primer layer
formed on the surface of the steel sheet. The two layers above them
are layers of coats formed by multilayer simultaneous coating. It
was observed that the interface between the upper and lower layers
formed by multilayer simultaneous coating forms a gentle wavy
shape. In FIG. 4B, the three layers are enlarged. The interface,
seen at the center, between the lower layer of the coats of the
multilayer simultaneous coating and the upper layer seen above it
showed roughness finer than the waviness in FIG. 4A. The white
spots in the lower layer of the coats of the multilayer
simultaneous coating are particles of pigment added to the
paint.
The fine roughness of the interface between the upper and lower
coat layers formed by the multilayer simultaneous coating can be
defined by finding the centerline average roughness Ra (JIS B 0601)
used for expressing the surface roughness of a material. That is,
it is possible to find the Ra by setting the curve of the interface
105 when viewed under .times.5000 magnification as a roughness
curve.
For example, it is possible to cut the coated steel sheet, embed it
in a resin, then polish it to smooth the cross-section
perpendicular to the surface of the coats, photograph this by a
.times.5000 scan type microscope, then find the Ra of the
interface. Specifically, it is possible to cover the photo with a
transparent sheet used for OHP, precisely trace the roughness of
the interface, then measure the area of the part of the vertical
lines as shown in FIG. 5 by an image processing system, and find
the average value from the following equation:
Ra=(.intg..sub.0.sup.l|f(x)|dx)/l In the equation, l is the
measured length in the centerline direction shown in FIG. 5.
To further simply measure the Ra of the interface, it is also
possible to cover the photograph by a transparent sheet as used for
OHP, precisely trace the roughness of the interface, draw an
average line corresponding to the centerline of FIG. 5, cut the
transparent sheet along the roughness, measure the weights of the
valley parts and peak parts above and below the average line, and
convert the weights to average length to find the Ra.
By examining the roughness of the interface of the coat layers
formed by multilayer simultaneous coating in this way, it was
learned that the value of the centerline average roughness Ra is in
general 0.3 .mu.m or more and even at a minimum about 0.25 .mu.m.
It was learned that the upper limit was in general about 0.7 .mu.m.
This relationship is similarly observed even in three or more
layers of coats formed by multilayer simultaneous coating. In this
case, the centerline average roughness Ra of the interface between
two adjoining layers observed under a high magnification of
.times.5000 also was generally 0.3 .mu.m or more and at the lowest
0.25 .mu.m or so. The upper limit was in general 0.7 .mu.m or so.
On the other hand, in the case of multilayer coats comprised of
coat layers formed by the conventional single layer coating method
superposed on each other, the Ra of the interface is 0.15 to 0.25
.mu.m or so. If examining this by a .times.5000 micrograph, the
difference from the coat layer according to the present invention
is clear.
The waviness of the interface between the upper and lower coat
layers as observed by a .times.500 low magnification enlarged
photograph of the cross-section of the precoated steel sheet of the
present invention produced by multilayer simultaneous coating is
believed to be due to these paint films both being in the undried
liquid state right after application as a result of application of
the two layers of paint superposed in the wet state. By studying
the interface with this waviness, it was learned that if the
maximum height H measured from the centerline C of the peaks 111
and valleys 113 of the waviness of the interface 105 shown in FIG.
3A is over 50% of the thickness t of the upper layer expressed as
the distance from the centerline C to the top surface of the upper
layer, mottled-like tone can be observed. In three or more layers
of coats formed by multilayer simultaneous coating, it is essential
that this relationship be satisfied between the layer positioned at
the topmost layer in the colored layers and the lower layer
adjoining this. It is preferable that in three or more layers of
coats, a similar relationship be satisfied between the clear top
coat layer formed together by multilayer simultaneous coating on
the layer positioned at the topmost layer in the colored layers and
the colored layer below it.
In the present invention, the thickness t expressed as the distance
from the centerline of the interface with waviness to the top
surface of the upper layer is made the average thickness of the
upper layer. Similarly, the thickness expressed as the distance
from the bottom surface of the lower layer to the centerline of the
interface with the upper layer is made the average thickness of the
lower layer. The average thickness of the intermediate layers other
than the topmost layer and bottommost layer in the case where the
coats formed by the multilayer simultaneous coating are comprised
of three or more layers is found as the distance between the
centerlines at the interfaces with the upper and lower layers with
waviness.
As shown in FIG. 6, when the lower layer 101 includes the pigment
107 and part of the pigment 107 is present at the parts of the
peaks 111 of the interface 105 with waviness, the top part of the
pigment 107 sometimes contacts the interface 111 or sticks out from
the interface 111 into the upper layer 103. The above maximum
height H when the pigment 107 sticks out from the interface 111 is
determined by viewing the contour 107a of the part of the pigment
107 sticking out from the interface 111 as the interface.
Next, the adhesion between coats is evaluated by examining the
presence of any peeling of the interfaces by the usual coin scratch
test method. By examining the relationship between the evaluated
adhesion between the coats and the roughness of the interface
measured by the Ra, it was learned that in a sample with an Ra of
the interface of less than 0.3 .mu.m, the adhesion between coats
was inferior. That is, even with coats of simultaneous multilayer
coating, if the Ra of the interface is less than 0.3 .mu.m, the
adhesion between coats became substantially the same as that of
multilayer coats obtained by stacking conventional single layer
coats prepared by the same paints. Both when introducing the
leveler into only the upper layer and when adding the leveler to
both the upper layer and lower layer, if the Ra is less than 0.3
.mu.m, it was learned that the adhesion is no longer good.
Further, the inventors investigated the relationship between the
difference in surface tensions of the lower layer and upper layer
and the roughness of the coat interface as expressed by Ra,
whereupon they learned that if the difference in surface tensions
of the lower layer and upper layer becomes larger, the indicator Ra
of the roughness of the coat interface becomes smaller. Further,
the difference in surface tensions of the lower layer and upper
layer when the Ra of the interface becomes less than 0.3 .mu.m was
observed to be 5 mN/m or more when introducing the leveler into
only the upper layer and 3.7 mN/m or more when adding the leveler
to both the upper layer and lower layer.
As explained above, as a condition for preventing mottled-like tone
and obtaining a score of 5, the difference in surface tensions of
the lower layer and upper layer has to be 1.2 mN/m or more when
introducing the leveler into only the upper layer and 0.3 mN/m or
more when adding the leveler to both the upper layer and lower
layer.
Therefore, when introducing the leveler into only the upper layer,
making the difference in surface tensions of the lower layer and
upper layer 1.2 mN/m to less than 5 mN/m was made a condition for
being able to achieve both prevention of mottled-like tone and
adhesion. Further, when adding the leveler to both the upper layer
and lower layer, making the difference in surface tensions of the
lower layer and upper layer 0.3 mN/m to less than 3.7 mN/m was made
a condition for being able to achieve both prevention of
mottled-like tone and adhesion.
The coats of the precoated metal sheet of the present invention are
formed using a multilayer simultaneous coating system. A schematic
view of a representative type of such a system as constituted by a
slide hopper type curtain coating system is shown in FIG. 7.
Referring to FIG. 7, a slide hopper 1 is provided with paint feed
ports 8 and slits 6 from which three layers of paints are
quantitatively fed out by a gear pump (not shown). A chain like
curtain guide 3 is provided so as to contact the two ends of the
lip part 7A of the slide surface 7. Below the lip part 7A, a paint
pan 5 is provided. The curtain guide 3 hangs down to the bottom of
the coating pan 5. The paint P is supplied from the paint feed
ports 8 of the slide hopper 1 through the slits 6 to the slide
surface 7 uniformly in the width direction and is laid on the slide
surface 7. When laid paint drop down from the front end (lip part
7A) of the slide surface 7 to the paint pan 5, it is spread by the
curtain guide 3, so flows down as a liquid film uniform in the
width direction as a curtain 4 of the paint. By passing a strip
like metal sheet, for example steel strip 2, through this liquid
film, it is possible to simultaneously coat a plurality of layers
of paint on the surface of the steel strip 2.
If using a slide hopper type curtain coating system, since a
plurality of layers are simultaneously coated without contact with
the surface of the metal sheet, there is none of the ridging which
cannot be avoided with a roll coater. Further, since the coat film
is made of a plurality of layers of curtains 4, so long as the
total thickness of the curtains 4 is a stable thickness or more,
that is, about 20 .mu.m in terms of dried thickness, it is possible
to coat a thickness of a single layer of paint of several .mu.m.
Therefore, by simultaneously coating the lower layer coat and the
upper layer coat, it is possible to obtain a coated metal sheet
with a beautiful appearance free of any ridging.
Further, by making the surface tension of the upper layer paint
lower than the surface tension of the lower layer paint, the upper
paint film will become smooth. This is because when the surface
tension of the upper layer paint is lower than that of the lower
layer paint, since the lower layer paint is constrained by the
surface of the metal sheet, a smoother surfacemost paint is
stabler, by the laws of fluid dynamics.
In precoated steel sheet preventing mottled-like tone and improved
in adhesion of the present invention, it is possible to use any
paint so long as the above conditions are satisfied. For example,
in the paint for forming the lower layer coat of the precoated
steel sheet, as the resin ingredient forming the coat, it is
possible to suitably use a polyester based resin. In addition to
this, the paint of the lower layer may also include various
ingredients generally used in paint compositions. For example, a
melamine resin-based, isocyanate-based, or other known curing agent
may be used. The lower layer paint may also contain, in accordance
with need, an extender pigment, aggregate, etc. As the solvent of
the coating, a hydrocarbon-based one, Anone/Sorbesso (1:1 mixed
solvent of cyclohexanone (usually called Anone) and Sorbesso 150),
etc. may be used.
In the upper layer paint as well, as the resin ingredient forming
the coat, again a polyester based resin may be used. In addition,
for example, a fluorine-based, acryl-based, silicone
polyester-based, urethane-based, epoxy-based, or other resin
ingredient may be used. The upper layer paint may include, in
addition to the polyester-based resin of the main resin, a melamine
resin-based, isocyanate-based, or other known curing agent. Again,
in addition, in accordance with need, an extender pigment,
defoamer, wax, etc. may be included. In the upper layer paint, a
general hydrocarbon-based solvent is used.
As the coloring pigment in the precoated metal sheet of the present
invention, it is possible to use any pigment selected from known
coloring pigments. Listing representative examples of coloring
pigments, there are organic pigments such as Azo Yellow,
Isoindoline Yellow, Azo Red, Phthalocyanine Blue, Suren Blue,
Quinacridone Red, etc. and inorganic pigments such as carbon black,
red oxide, yellow lead, Molybdate Orange, Titanium White,
Ultramarine Blue, Prussian Blue, Titanium Yellow, graphite, zinc
white, etc.
For drying and baking the paint, for example, a hot air oven,
induction furnace, near infrared furnace, far infrared furnace, or
energy beam curing furnace may be used. From the viewpoint of the
balance between the evaporation of the solvent due to heating or
curing of the paint and the deterioration of the resin ingredient
in the paint, the baking temperature of the paint is preferably
150.degree. C. to less than 320.degree. C.
In the top clear type precoated metal sheet superior in weather
resistance of the present invention, the upper layer of the coat
layer is a transparent or semitransparent clear layer, and the
centerline average roughness Ra of the interface between the lower
coat layer adjoining the upper clear layer and the upper clear
layer is 0.3 to 0.7 .mu.m. Further, the Ra of the interface of 0.3
to 0.5 .mu.m is preferable in giving a feeling of weight to the
appearance.
In the top clear type precoated metal sheet of the present
invention, the interface Ra can be adjusted as explained earlier
utilizing a leveler (a type of surfactant) to control the
difference in surface tensions between the upper layer and lower
layer. The leveler able to be used for this was as explained
before. Again, as explained earlier, it is believed that by the
surface tension of the lower layer being larger than the surface
tension of the upper layer by the amount of the interfacial
tension, the upper layer paint easily spreads on the lower layer
paint, the force disturbing the interface between the upper layer
and lower layer due to Marangoni convection and broadening the
interface is suppressed, and abnormal disturbance of the interface
accompanying application of the paints of the upper and lower coats
in the liquid state is suppressed.
As already explained, regarding the relationship between the
difference in surface tensions of the lower layer and upper layer
and the fine roughness at the interface of the coat layers
(centerline average roughness Ra), it was learned that as the
difference in surface tensions of the lower layer and the upper
layer becomes larger, the Ra indicator of the roughness at the
interface of the coats becomes smaller. The difference in surface
tensions between the lower layer and the upper layer when the Ra of
the interface is less than 0.3 .mu.m was 5 mN/m or more when
introducing the leveler into only the upper layer and was 3.7 mN/m
or more when introducing the leveler to the upper layer and the
lower layer. Therefore, in the present invention, to achieve an Ra
of 0.3 .mu.m or more at the interface of the upper layer and lower
layer, it is sufficient to make the difference in surface tension
between the lower layer and upper layer less than 5 mN/m when
introducing the leveler to only the upper layer and to less than
3.7 mN/m when introducing the leveler to both the upper layer and
the lower layer.
The surface tension and Ra of the interface mentioned here may be
measured by the methods explained above.
The top clear type precoated metal sheet of the present invention
is a precoated metal sheet giving a color, pattern, design, etc. to
the upper clear layer so that the color, pattern, design, etc. of
the base coat can be seen and thereby impart a design nature rich
in variety by a synergistic effect between the lower coat layer and
upper clear layer. Since the upper clear layer is transparent
(clear) to an extent enabling the lower coat layer to be seen
through the upper clear layer (top coat), it is called a "top clear
type". The lower coat layer is not particularly limited in color,
pattern, or design and may be obtained using any one or combination
of known techniques.
To give the top clear layer a synergistic effect with the lower
coat as explained above, it is necessary that the color, pattern,
design, etc. of at least the substrate be visible through the top
clear layer. If satisfying this condition, it is possible to use a
pigment, dye, aggregate, or delustering agent or other known
technique used for paints to give the top clear layer a color,
pattern, or design. That is, the top clear layer need only have a
transparency of an extent where the color etc. of the lower coat
layer is substantively reflected in the appearance of the coated
metal sheet (that is, may also be semitransparent).
Further, an additive for imparting lubrication, stain resistance,
or other functions, an additive for improving the paint work, the
dispersion stability of the pigment etc., etc., a UV absorbent for
improving the UV resistance, an antioxidant, or another additive or
another ingredient may be added to the top clear layer. Needless to
say, it is also possible not to blend any pigment etc. in the top
clear layer and secure a completely clear state.
It is further possible to form a coat layer over this top clear
layer so as to protect this top clear layer or further obtain a
synergistic effect by multiple layers. In this case as well, it is
necessary that the lower layer coat be visible through the
superposed coat.
In the top clear type precoated metal sheet of the present
invention, when the base metal sheet is steel sheet, for example,
stainless steel sheet and plated steel sheet may be suitably used.
As stainless steel sheet, ferritic stainless steel sheet,
martensitic stainless steel sheet, austenitic stainless steel
sheet, etc. may be mentioned. As plated steel sheet, galvanized
steel sheet, zinc-iron alloy plated steel sheet, zinc-nickel alloy
plated steel sheet, zinc-chrome alloy plated steel sheet,
zinc-aluminum alloy plated steel sheet, aluminum-silicon alloy
plated steel sheet, zinc-aluminum-magnesium alloy plated steel
sheet, zinc-aluminum-magnesium-silicon alloy plated steel sheet,
zinc-plated stainless steel sheet, aluminum-plated stainless steel
sheet, etc. may be mentioned.
As the treatment before coating of the steel sheet, there are cold
water washing, hot water washing, pickling, alkali degreasing,
grinding, polishing, etc. It is possible to perform these alone or
in combination in accordance with need. The conditions of the
treatment before coating also may be suitably selected. Before
coating the enamel base coat, it is also possible to
conversion-treat the surface in accordance with need. It is also
possible to provide a primer layer having a rust preventing pigment
on the conversion-treated layer and provide an enamel base coat
thereon. That is, it is also possible to provide a top clear type
precoated steel sheet of a three-layer structure of an undercoat
layer, enamel pigment-containing midcoat layer, and topcoat (clear)
layer.
The upper clear layer is a transparent or semitransparent clear
layer. The paint for forming the upper clear layer is not
particularly limited, but for example a polyester, acryl, silicone,
fluorine, urethane, or olefin type or mixtures or copolymers of the
same may be suitably used.
The paint of the upper clear layer may also contain, in accordance
with need, a cross-linking agent such as an amino resin, isocyanate
resin, epoxy resin, or other known agent. Further, as explained
above, it is also possible to include a pigment or other coloring
agent within a range where the lower layer can be seen.
Further, in accordance with need, it is also possible to utilize an
upper clear layer described in JP 2003-326639 A or JP 10-193509
A.
For example, the coating for the upper clear layer may also have
added to it, within a range not impairing the transparency and for
the purpose of imparting the design property, pearl, mica, metal
powder (aluminum powder, nickel powder, stainless steel powder,
etc.), organic enamel beads (urethane resin beads, acryl resin
beads, etc.), or coloring pigments or dyes.
In the present invention, the coats to be formed on the metal sheet
include at least the above-mentioned transparent or semitransparent
upper clear layer and lower coat layer adjoining that upper clear
layer. In accordance with need, it may also include layers other
than these upper clear layer and lower coat layer (for example, a
layer between the lower coat layer and metal sheet).
The paint for forming the lower coat layer is not particularly
limited, but for example a polyester, acryl, silicone, fluorine,
urethane, or olefin types or mixtures or copolymers of the same may
be suitably used.
The paint of the lower coat layer may also contain, in accordance
with need, a cross-linking agent such as an amino resin, isocyanate
resin, epoxy resin, or other known agent. The lower coat layer
preferably contains a pigment or other coloring agent (for example,
when there is a still other coat between the lower coat layer and
the metal sheet). Further, an additive for imparting lubrication,
stain resistance, or another function, an additive for improving
the painting work efficiency, the dispersion stability of the
pigment etc., a UV absorbent for improving the UV resistance, an
antioxidant, or other additive or other ingredient may also be
added to the lower layer.
Further, in accordance with need, it is also possible to utilize a
material for the lower coat layer described in JP 2003-326639 A or
JP 10-193509 A.
For example, as the "enamel base coat" forming one embodiment of
the lower coat layer in the present invention, it is possible to
use an enamel base coat or precoated steel sheet commonly used as
it is.
As an enamel base coat, a polyester resin-based base coat, acryl
resin-based base coat, silicone polyester resin-based base coat,
PET resin-based base coat, urethane resin-based base coat,
polyvinyl chloride resin-based base coat, fluorine resin-based base
coat, etc. may be mentioned. When further superior weather
resistance is necessary, it is possible to use a resin the same as
that used for the clear paint of the present invention for the base
coat coating. That is, this may be achieved using an enamel base
coat paint obtained by blending into a copolymerized acryl polyol
obtained by copolymerizing one or both of a polymerizable UV stable
monomer and cycloalkyl group-containing polymerizable monomer and a
polymerizable monomer ingredient essentially including a
hydroxy-group containing polymerizable monomer a cross-linking
agent comprised of an aminoplast resin or polyisocyanate compound,
and dissolving or dispersing the blend in a solvent. Further, by
adding a UV stabilizer, it is possible to impart further superior
weather resistance. As a UV stabilizer, as explained above, a
benzophenone-based, benzotriazole-based, anilide succinate-based,
cyanoacrylate-based, triazine-based, or other stabilizer may be
used. These may be used as additive or may be used in a form
chemically bonded with the resins of the present invention.
The top clear type precoated metal sheet of the present invention
is produced by simultaneously coating at least one surface of a
metal sheet with a lower layer coat layer (for example, an enamel
base coat) and a coat layer of a clear paint utilizing a multilayer
simultaneous coating system and then drying and baking the same.
The paints may be dried and baked by heating utilizing a hot air
oven, induction furnace, near infrared furnace, far infrared
furnace, or energy beam curing furnace. From the viewpoint of the
balance of the evaporation of the solvent or curing of the paints
and the deterioration of the resin ingredient in the paints
(appearance and workability as precoated metal sheet), the baking
temperature of the paints is preferably 150.degree. C. to less than
320.degree. C.
In the top clear type precoated metal sheet of the present
invention, so long as the coats arranged on the metal sheet include
at least an upper clear layer and lower coat layer and the
centerline average roughness Ra of the interface between the upper
clear layer and lower coat layer is 0.3 to 0.7 .mu.m, there may be
three or more layers of coats.
The thicknesses of the coated films of the top clear type precoated
metal sheet of the present invention are not particularly limited.
That is, the suitable thicknesses may also differ depending on the
coat performance or applications, so these are preferably selected
discretionally. However, this being said, in general, the thickness
of the enamel base coat is preferably 5 .mu.m to 40 .mu.m or so,
while the thickness of the clear coat layer is preferably 1 to 40
.mu.m.
In the present invention, when including a curing agent (for
example, a melamine resin) in all or part of the coats including at
least the two layers of the lower coat layer and upper clear layer,
it is possible to introduce into each individual coat a curing
agent for the resin forming that coat.
Further, in the present invention, it is also possible to include a
curing agent for another coat in the coats in addition to or in
place of the curing agent for the resins forming the coats. That
is, in the present invention, for example, it is possible to
combine a curable resin and a curing agent as follows (in the
following table, the curing agent for the resin A being referred to
as "a", the curing agent for the resin B as "b", and the curing
agent for the resins A and B in common as "c").
TABLE-US-00001 Curing agent Curing agent Lower coat contained in
Upper clear contained in layer lower layer layer upper layer (1)
Resin A Curing agent a Resin B Curing agent b (2) Resin A Curing
agent c Resin B None (3) Resin A None Resin B Curing agent c (4)
Resin A Curing agent c Resin B Curing agent c
When adding a curing agent reacting with a paint resin of another
layer to one or both of the upper clear layer and lower coat layer,
it is possible to further improve the adhesion between layers based
on the mutual dispersion of the curing agent (for example, melamine
resin) at the interface of the upper clear layer/lower coat
layer.
In the present invention, using the above method (or any other
method) to provide a gradient of concentration of the curing agent
at least at the part in the vicinity of the interface of the lower
coat layer and upper clear layer is preferable from the viewpoint
of further improving the adhesion between these layers. This is
also preferable from the viewpoint that the presence of a gradient
of concentration of the curing agent at the part in the vicinity of
the interface results in a change of the refractive index of light
in the vicinity of the interface and a further increase in the
ratio of the irregular reflection at the interface.
The precoated metal sheet achieving both workability and corrosion
resistance of the present invention is characterized by having a
lower coat layer and an upper coat layer thereon formed on the
metal sheet by multilayer simultaneous coating, wherein the lower
coat layer contains an average 30 wt % or more of a rust preventing
pigment, the upper coat layer contains a rust preventing pigment
the same as the rust preventing pigment in the lower layer coat by
an average content lower than the average content of the lower coat
layer and by a concentration gradient gradually decreasing the
further from the vicinity of the interface with the lower coat
layer, and the centerline average roughness Ra at the interface
between the lower coat layer and upper coat layer is 0.3 to 0.7
.mu.m.
The paint forming the lower coat layer of the precoated steel sheet
contains a resin ingredient for forming a coat and a rust
preventing pigment effective for suppression of corrosion of the
steel sheet. As the resin ingredient for forming the coat, it is
possible to suitably use a polyester-based resin. The paint of the
lower coat layer may also include various ingredients generally
used in paint compositions. For example, it is possible to use a
melamine resin-based, isocyanate-based, or other known curing
agent. The paint of the lower coat layer may also contain in
accordance with need an extender pigment or aggregate and further
may contain titanium white etc. as the coloring pigment. As the
solvent of the paint, it is possible to use the hydrocarbon-based
Anone/Sorbesso (1:1 mixed solvent of cyclohexanone (usually known
as Anone) and Sorbesso 150) etc. The ratio of mixture of the main
resin and the other ingredients may be suitably determined by the
coating conditions of the paints, the requirements of the coats
formed, etc. For example, when using a melamine resin-based curing
agent, it is possible to use 5 to 40 parts by mass of the curing
agent with respect to 100 parts by mass of the main resin. Further,
the solvent may be used in an amount of 50 to 200 parts by mass and
the rust preventing pigment in an amount of 20 to 100 parts by mass
with respect to 100 parts by mass of the main resin.
Typical examples of the rust preventing pigment contained in the
lower coat layer of the precoated steel sheet according to the
present invention include chromate-based compounds such as
strontium chromate and barium chromate, and nonchrome-based rust
preventing pigments such as calcium silicate, and phosphate-based
compounds, and the like.
In the paint forming the upper coat layer as well, it is possible
to use, as the resin ingredient forming the coat, a polyester-based
resin similar to that explained previously for a paint forming the
lower coat layer. In addition, for example, it is also possible to
use a fluorine-based, acryl-based, silicone polyester-based,
urethane-based, epoxy-based, or other resin ingredient. The paint
forming the upper coat layer may also include, in addition to the
polyester-based resin of the main resin, a melamine resin-based,
isocyanate-based, or other known curing agent. In addition, the
paint of the upper layer coat may also include in accordance with
need an extender pigment, defoamer, leverer, wax, etc. In the paint
of the upper coat layer, in general a hydrocarbon-based solvent is
used. The ratio between the main resin and the other ingredients
may be suitably determined in accordance with the conditions of
application of the paint, the requirements of the coat formed, etc.
For example, when using a melamine resin-based curing agent, it is
possible to use 5 to 100 parts by mass of a curing agent with
respect to 100 parts by mass of the main resin. Further, it is also
possible to use a solvent in an amount of 50 to 200 parts by mass
and a wax in an amount of 0.5 to 5 parts by mass with respect to
100 parts by mass of the main resin.
The above-mentioned ingredients of the paint used in the precoated
steel sheet achieving both workability and corrosion resistance
according to the present invention are all widely known and are not
special.
In the precoated metal sheet achieving both the workability and
corrosion resistance of the present invention, the lower coat layer
contains an average of 30 wt % or more of a rust preventing
pigment, the upper coat layer contains the same rust preventing
pigment as the rust preventing pigment in the lower coat layer by
an average content lower than the average content of the lower coat
layer and by a concentration gradient gradually decreasing the
further from the vicinity of the interface with the lower coat
layer. The rust preventing pigment in the upper coat layer is
mainly present near the interface with the lower coat layer and
should not be detected at the surface of the upper coat layer. If
the rust preventing pigment is present at the surfacemost layer of
the coated metal sheet, the stain resistance, workability,
appearance, and color tone become poor.
The lower coat layer and upper coat layer in the precoated metal
sheet achieving both workability and corrosion resistance of the
present invention can be formed by including in the paint for the
lower layer formed on the metal sheet a greater amount of rust
preventing pigment than the average content of rust preventing
pigment targeted in the lower layer of the completed coated metal
sheet and using the heat when baking the paint film for the lower
layer and the paint film for the upper layer on the metal sheet
after coating so as to cause the rust preventing pigment in the
lower layer to disperse to the upper layer. The amount of the rust
preventing pigment to be initially included in the paint for the
lower layer can be simply determined through experiments based on
the target content of the rust preventing pigment in the lower
layer of the precoated metal sheet, the baking conditions, etc.
The average content of the rust preventing pigment in the lower
coat layer of the precoated metal sheet is at least 30 wt %. When
less than 30 wt %, the precoated metal sheet is no longer
sufficiently improved in the corrosion resistance. Preferably, the
average content of the rust preventing pigment content in the lower
coat layer is 50 wt % or more. When introducing the rust preventing
pigment in the lower coat layer, conventionally the upper limit of
the rust preventing pigment content is about 40 wt %. The reason is
that even if increasing the rust preventing pigment more than that,
the effect ends up being saturated. Further, if increasing the
amount of the rust preventing pigment, the lower coat layer becomes
brittle and no improvement in the corrosion resistance can be
expected any longer. However, in the precoated metal sheet of the
present invention, due to the effect of the continuity of the
distribution of hardness in the thickness direction of the coat
explained later, it is possible to raise the corrosion resistance
in accordance with the content of the rust preventing pigment in
the lower coat layer even if the lower coat layer becomes
brittle.
In a precoated metal sheet of the present invention achieving both
workability and corrosion resistance, it is also important that the
centerline average roughness Ra of the interface between the lower
coat layer and upper coat layer be 0.3 to 0.7 .mu.m (the Ra being
measured as explained above). By making the Ra of the interface
this range, the adhesion of the upper coat layer and lower coat
layer can be secured and the workability of the coat layers can be
improved. If the Ra of the interface is less than 0.3 .mu.m, the
adhesion between the upper coat layer and the lower coat layer is
insufficient and the workability of the coat layers is impaired.
The upper limit of 0.7 .mu.m is the upper limit of Ra usually
observed at adjoining layers of multilayer simultaneous
coating.
The centerline average roughness Ra in the above range of the
interface between the lower coat layer and upper coat layer can be
obtained by utilizing the method of using, for example, a slide
coater or other coater able to simultaneously form two or more
layers of coats to simultaneously coat upper layer and lower layer
paint films on the metal sheet and then simultaneously baking them.
A slide coater has the slide surface where the discharge port of
the paint is located and the metal sheet in the form of a coated
strip separated from each other, so the paint films are resistant
to the effects of the discharge flow of the paints and enable the
thickness of the coat layers in the width direction and the state
of Ra to be maintained uniform. As explained earlier, the Ra of the
coat layer interface can be controlled by utilizing the difference
in surface tensions of the paints forming the upper and lower coat
layers. The surface tension of a paint is determined from the
ingredients and composition of the paint, so it is possible to
adjust these to adjust the Ra of the interface. This being said, as
explained above, it is preferable to utilize a leveler additive to
adjust the Ra of the interface.
In this way, the precoated metal sheet of the present invention
achieving both workability and corrosion resistance can be produced
by the method of making part of the rust preventing pigment in a
lower coat layer disperse in an upper coat layer when baking a
paint film comprised of a lower coat layer containing a rust
preventing pigment coated on the surface of a metal sheet and an
upper coat layer thereon using a multilayer simultaneous coating
system, wherein the average concentration of the rust preventing
pigment in the lower layer formed by baking becomes 30 wt % or
more, the upper layer contains a rust preventing pigment the same
as the rust preventing pigment of the lower layer by an average
content lower than the average content of the lower layer and by a
concentration gradient gradually decreasing the further from the
vicinity of the interface with the lower layer, a leveler is added
to the upper coat layer or the upper coat layer and lower coat
layer, and a centerline average roughness Ra of the interface
between the lower coat layer and upper coat layer becomes 0.3 to
0.7 .mu.m.
In general, if subjecting a sample of a precoated metal sheet
provided with an upper coat layer and a lower coat layer to a
saltwater spray test (SST), in a sample inferior in corrosion
resistance, as shown in FIG. 8, blisters 14 and 16 due to the
saltwater invading the interface between the upper coat layer and
the lower coat layer are observed in the region in the cross-cut
part 13 of the coating of the sample 11 of an extent of a width of
1 mm or so from the cross-cut line 13a along the line 13a and the
region of an extent of a width of 5 mm or so from the end face.
This blistering is believed to occur due to the following
mechanism.
At the cross-cut part, as shown in FIG. 9, when making a cut into
the lower coat layer 22 on the metal sheet 21 and the upper coat
layer 23 by a knife 25, strain parts 27 as illustrated occur in the
lower coat layer 22 and upper coat layer 23. On the other hand, at
the end faces of the precoated metal sheet, as shown in FIG. 10,
the illustrated strain parts 35 occur in the lower coat layer 32
and the upper coat layer 33 on the metal sheet 31 at the time of
cutting the precoated metal sheet. After this, the lower coat layer
22 and upper coat layer 23 of the cross-cut part and the lower coat
layer 32 and the upper coat layer 33 of the end faces return to
their original strain-free states. At this time, if the adhesions
between the lower coat layers 22, 32 and the upper coat layers 23,
33 are good, saltwater does not easily penetrate at the interface
of the two layers and corrosion resistance is secured, but when the
adhesion of the two layers is poor, saltwater penetrates through
their interface to cause blisters of the coat layers, that is, the
precoated metal sheet exhibits insufficient corrosion
resistance.
In the precoated metal sheet of the present invention, by
introducing a 30 wt % or more relatively high content of rust
preventing pigment into the lower coat layer, expression of the
corrosion resistance by the lower coat layer is secured. On the
other hand, by making the upper coat layer also include the rust
preventing pigment dispersed from the lower coat layer in the
production process by a suitable concentration gradient and making
the centerline average roughness Ra of the interface between the
lower coat layer and upper coat layer 0.3 to 0.7 .mu.m, the
adhesion of the upper coat layer and lower coat layer is improved
and thereby the workability of the metal sheet is secured and the
drop in corrosion resistance due to permeation of moisture at the
interface between the lower coat layer and upper coat layer is
prevented.
In general, the coatings of a two-layer structure of a precoated
metal sheet are formed so that the upper layer is hard and the
lower layer is soft. If including in the upper layer of the coating
formed by baking two layers of paints a rust preventing pigment the
same as that of the lower layer by a suitable concentration
gradient, the distribution of the hardness in the thickness
direction of the coating becomes continuous (or substantially
continuous). Due to this, at the lower layer, which is inherently
soft and becomes brittle due to the inclusion of the rust
preventing pigment and so is susceptible to cracks, the cracks
occurring at the time of working the metal sheet become fewer.
Therefore, even if increasing the rust preventing pigment and the
lower layer becomes further brittle, a sufficient corrosion
resistance can be obtained. When separately baking the upper coat
layer and lower coat layer, the change in hardness of the layers at
the interface of the two layers becomes discontinuous, destruction
easily occurs there, and both the workability and the corrosion
resistance are lowered. Further, in the present invention, making
the centerline average roughness Ra at the interface of the lower
coat layer and upper coat layer 0.3 to 0.7 .mu.m to secure adhesion
between the upper coat layer and lower coat layer also contributes
to securing the corrosion resistance.
The method of baking the paint film of the coated multilayer
structure is not particularly limited. For example, hot air,
induction heating, etc. may be utilized. A plurality of methods may
also be used together. In the case of an irradiation curing type
coat layer, it is also possible to simultaneously use irradiation.
From the viewpoint of the productivity, while not as preferable as
baking by forced drying by heating, the coat layers may also be
naturally dried. In the case of heating, the metal sheet is
generally raised in temperature to 40 to 250.degree. C.
The suitable thickness of the upper coat layer is, in terms of
average thickness, 10 to 30 .mu.m. If thinner than 10 .mu.m, the
stain resistance cannot be sufficiently secured, while if thicker
than 30 .mu.m, the layer is inferior in economy and further the
workability may drop. A more preferable thickness of the upper coat
layer is 12 to 20 .mu.m.
The thickness of the lower coat layer may be suitably determined
from the workability and, in some cases, the corrosion resistance
and other aspects of the performance, but in general the suitable
thickness is 2 to 15 .mu.m. If thinner than 2 .mu.m, the layer is
inferior in corrosion resistance, while if thicker than 15 .mu.m,
it is inferior in economy and also drops in workability in some
cases. A more preferable thickness of the lower coat layer is 5 to
10 .mu.m.
The average thickness may be measured using the above method
utilizing an enlarged photograph of the cross-section. Any other
method may also be used. For convenience, the weight method may be
utilized (though it is necessary to separately form and measure the
lower coat layer and upper coat layer).
Examples will be used to further explain the present invention, but
the present invention is not limited to these examples.
EXAMPLES
Example 1
The present invention was applied to the case when using the
production treatment line for a precoated steel sheet shown in FIG.
11 to curtain coat a multilayer film on a steel strip.
In the facility of FIG. 11, the steel strip wound in a coil was
uncoiled by an uncoiler 41 and passed through an accumulator 42,
conversion treatment system 47, prime coater 45, and induction
furnace 43. At a later position, a slide hopper type curtain
coating system 49 of slide type is disposed to curtain coat the
surface of a running steel sheet 11 with a multilayer film.
Downstream of the curtain coating system 49, an induction furnace
51 was provided as a facility for drying and baking the coated
paints. After this, the steel sheet is passed through an
accumulator 53 and is coiled up by a recoiler 44 as a finished
treated steel strip.
The slide hopper type curtain coating system 49 was used for
simultaneous coating of two layers. The slide hopper type curtain
coating system had a size of a slit width on the coater of 200 mm,
an interval of slits of 500 .mu.m, and a height to the steel sheet
being coated of 150 mm. The steel sheet was moved under the
multilayer slide coater where multilayer coats were formed on the
steel sheet. After two layers of coats were simultaneously formed,
they were dried and baked by an induction furnace. The lower layer
had a thickness of 2 to 15 .mu.m and the upper layer had a
thickness of 0.5 to 15 .mu.m. The heating rate of the induction
furnace was made 2 to 10.degree. C./s and the peak metal
temperature of the steel sheet after baking was made 200 to
230.degree. C.
The next paints were used.
Paint 1: High molecular weight polyester/isocyanate system
Paint 2: High molecular weight polyester/melamine system
Paint 3: High molecular weight polyester/melamine system
As the pigments added to the paints, the following were used.
White pigment: Titanium oxide pigment
Red pigment: Iron oxide pigment
Gray metallic pigment: Prepared by combining various types of
pigments to obtain a gray color, then adding flat aluminum
particles (long diameter of about 20 .mu.m).
Clear metallic pigment: Prepared by adding flat aluminum particles
(long diameter of about 20 .mu.m) to clear paint.
As the levelers, the following were used.
Leveler A: Acryl-based leveler (made by Nippon Paint)
Leveler B: Silicone-based additive BYK141 (made BYK-Chemie)
Leveler C: Nonsilicone-based additive BYK057 (made by
BYK-Chemie)
When coating the steel sheet with a primer, a nonchromate-based
primer Flexicoat 690 Primer (made by Nippon Fine Coatings) was
baked on to a dry thickness of 5 .mu.m.
The surface tension was measured using a Dynometer (BYK-Chemie
GmbH). Each paint was measured five times and the average value
calculated was used as the surface tension of the paint. A paint of
a difference between the maximum value and minimum value of the
five measurements of less than 2 mN/m was evaluated as "A" and a
paint of 2 mN/m or more was evaluated as "B".
The average thickness of the layers and the centerline average
roughness Ra of the interface, were measured using the curve of the
interface obtained by covering an .times.5000 scan type micrograph
of a cross-section of the coat layers by a transparent sheet used
for OHP and tracing it, as explained earlier. Further, the ratio of
the maximum height from the centerline of the waviness of the
interface as observed by .times.500 magnification to the thickness
measured from centerline of the layer positioned on the interface
was found as the waviness % of the interface.
The mottled-like tone score was expressed using no mottled-like
tone able to be seen as a score of 5 and mottled-like tone
occurring to an extent able to be sufficiently confirmed visually
as a score of 1 and dividing the interval between them into
different extents of occurrence of mottled-like tone, as explained
above. For example, a score of 4 indicates mottled-like tone which
cannot be clearly seen visually, but is of an extent which can be
confirmed by viewing by a .times.10 loupe.
The coin scratch test was used to evaluate the adhesion between the
coat layers. The coated surface of each steel sheet was scratched
by pressing a 100 yen Japanese coin against it so as to make a
scratch reaching the base sheet and evaluating the occurrence of
peeling at the interface of the two simultaneously coated layers at
the two ends of the scratch as "poor" and no occurrence as
"good".
The working heat adhesion between the coat layers was evaluated by
the following method. Each steel sheet was drawn into a cylindrical
cup by a draw ratio of 2.0 and a wrinkle suppressing pressure of
1.0 ton so that the evaluated surface became the outside. After
that, the cup was heated by a hot air oven at 120.degree. C. for 20
minutes, then taken out and observed for the coat appearance of the
cup side surface after cooling. A sample with peeling of the coats
or cracking of the coats was evaluated as "poor" and one with none
of the above as "good".
Table 1 and Table 2 show the results of the experiments. These
tables show only examples with primers, but the results were the
same for the cases with no primers as well.
TABLE-US-00002 TABLE 1 Lower layer Surface tension Average Upper
Average Viscosity Color thickness layer No. Primer Paint Leveler
value (mN/m) Fluctuation (sec) (pigment) (.mu.m) Paint 1 Ex. Yes
Paint 2 None 33.8 A 120 White 10 Paint 2 2 Ex. Yes Paint 2 None
33.8 A 120 White 2 Paint 2 3 Ex. Yes Paint 2 None 33.8 A 120 White
2 Paint 2 4 Ex. Yes Paint 2 None 33.8 A 120 White 5 Paint 2 5 Ex.
Yes Paint 2 None 33.8 A 120 White 5 Paint 2 6 Ex. Yes Paint 2 None
33.8 A 120 White 15 Paint 2 7 Comp. Ex. Yes Paint 2 None 33.8 A 120
White 10 Paint 2 8 Comp. Ex. Yes Paint 2 None 33.8 A 120 White 2
Paint 2 9 Comp. Ex. Yes Paint 2 None 33.8 A 120 White 2 Paint 2 10
Comp. Ex. Yes Paint 2 None 33.8 A 120 White 5 Paint 2 11 Comp. Ex.
Yes Paint 2 None 33.8 A 120 White 5 Paint 2 12 Comp. Ex. Yes Paint
2 None 33.8 A 120 White 15 Paint 2 13 Ex. Yes Paint 2 None 33.8 A
120 White 10 Paint 2 14 Ex. Yes Paint 2 None 33.8 A 120 White 2
Paint 2 15 Ex. Yes Paint 2 None 33.8 A 120 White 2 Paint 2 16 Ex.
Yes Paint 2 None 33.8 A 120 White 5 Paint 2 17 Ex. Yes Paint 2 None
33.8 A 120 White 5 Paint 2 18 Ex. Yes Paint 2 None 33.8 A 120 White
15 Paint 2 19 Ex. Yes Paint 2 None 33.8 A 120 White 10 Paint 2 20
Ex. Yes Paint 2 None 33.8 A 120 White 15 Paint 2 21 Ex. Yes Paint 2
None 33.8 A 120 White 15 Paint 2 22 Ex. Yes Paint 2 None 33.7 A 120
Gray 10 Paint 2 metallic 23 Ex. Yes Paint 2 None 33.7 A 120 Gray 2
Paint 2 metallic 24 Ex. Yes Paint 2 None 33.7 A 120 Gray 2 Paint 2
metallic 25 Ex. Yes Paint 2 None 33.7 A 120 Gray 5 Paint 2 metallic
26 Ex. Yes Paint 2 None 33.7 A 120 Gray 5 Paint 2 metallic 27 Ex.
Yes Paint 2 None 33.7 A 120 Gray 15 Paint 2 metallic 28 Ex. Yes
Paint 2 None 33.7 A 120 Gray 10 Paint 2 metallic 29 Ex. Yes Paint 2
None 33.7 A 120 Gray 15 Paint 2 metallic 30 Ex. Yes Paint 2 None
33.7 A 120 Gray 15 Paint 2 metallic 31 Ex. Yes Paint 2 None 33.8 A
120 White 10 Paint 2 32 Ex. Yes Paint 2 None 33.8 A 120 White 2
Paint 2 33 Ex. Yes Paint 2 None 33.8 A 120 White 2 Paint 2 Upper
layer Surface tension Average Surface tension Average Viscosity
Color thickness difference No. Leveler value (mN/m) Fluctuation
(sec) (pigment) (.mu.m) (lower-upper) 1 Ex. A 31.1 A 120 Red 15 2.7
2 Ex. A 31.1 A 120 Red 15 2.7 3 Ex. A 31.1 A 120 Red 10 2.7 4 Ex. A
31.1 A 120 Red 15 2.7 5 Ex. A 31.1 A 120 Red 10 2.7 6 Ex. A 31.1 A
120 Red 10 2.7 7 Comp. Ex. C 30.9 B 120 Red 15 2.9 8 Comp. Ex. C
30.9 B 120 Red 15 2.9 9 Comp. Ex. C 30.9 B 120 Red 10 2.9 10 Comp.
Ex. C 30.9 B 120 Red 15 2.9 11 Comp. Ex. C 30.9 B 120 Red 10 2.9 12
Comp. Ex. C 30.9 B 120 Red 10 2.9 13 Ex. A 31.1 A 120 Clear 15 2.7
14 Ex. A 31.1 A 120 Clear 15 2.7 15 Ex. A 31.1 A 120 Clear 10 2.7
16 Ex. A 31.1 A 120 Clear 15 2.7 17 Ex. A 31.1 A 120 Clear 10 2.7
18 Ex. A 31.1 A 120 Clear 10 2.7 19 Ex. A 31.1 A 120 Clear 5 2.7 20
Ex. A 31.1 A 120 Clear 1 2.7 21 Ex. A 31.1 A 120 Clear 0.5 2.7 22
Ex. A 31.1 A 120 Clear 15 2.6 23 Ex. A 31.1 A 120 Clear 15 2.6 24
Ex. A 31.1 A 120 Clear 10 2.6 25 Ex. A 31.1 A 120 Clear 15 2.6 26
Ex. A 31.1 A 120 Clear 10 2.6 27 Ex. A 31.1 A 120 Clear 10 2.6 28
Ex. A 31.1 A 120 Clear 5 2.6 29 Ex. A 31.1 A 120 Clear 1 2.6 30 Ex.
A 31.1 A 120 Clear 0.5 2.6 31 Ex. A 31.1 A 120 Clear 15 2.7
metallic 32 Ex. A 31.1 A 120 Clear 15 2.7 metallic 33 Ex. A 31.1 A
120 Clear 10 2.7 metallic Surface tension Lower Upper difference
Lower No. Primer layer layer (lower-upper) No. Primer layer 34 Ex.
Yes Paint 2 None 33.8 A 120 White 5 35 Ex. Yes Paint 2 None 33.8 A
120 White 5 36 Ex. Yes Paint 2 None 33.8 A 120 White 15 37 Ex. Yes
Paint 2 None 33.8 A 120 White 10 38 Ex. Yes Paint 2 None 33.8 A 120
White 10 39 Ex. Yes Paint 1 None 33.1 A 140 White 10 40 Ex. Yes
Paint 1 None 33.5 A 666 White 10 41 Ex. Yes Paint 1 None 33.5 A 666
White 10 42 Ex. Yes Paint 1 None 33.1 A 266 White 10 43 Ex. Yes
Paint 1 None 33.1 A 266 White 10 44 Ex. Yes Paint 1 None 33.1 A 266
White 10 45 Ex. Yes Paint 1 None 33.1 A 266 White 10 46 Ex. Yes
Paint 1 None 33.5 A 666 White 10 47 Ex. Yes Paint 1 None 33.5 A 666
White 10 48 Ex. Yes Paint 1 None 33.1 A 266 White 10 49 Ex. Yes
Paint 1 None 33.1 A 266 White 10 50 Comp. Ex. Yes Paint 1 None 33.1
A 266 White 10 51 Comp. Ex. Yes Paint 1 None 33.1 A 266 White 10 52
Comp. Ex. Yes Paint 1 None 33.1 A 140 White 10 53 Comp. Ex. Yes
Paint 1 None 33.1 A 140 White 2 54 Comp. Ex. Yes Paint 1 None 33.1
A 140 White 2 55 Comp. Ex. Yes Paint 1 None 33.1 A 140 White 5 56
Comp. Ex. Yes Paint 1 None 33.1 A 140 White 5 57 Comp. Ex. Yes
Paint 1 None 33.1 A 140 White 15 58 Comp. Ex. Yes Paint 1 None 31.4
A 200 White 10 59 Comp. Ex. Yes Paint 1 None 31.3 A 200 White 10 60
Comp. Ex. Yes Paint 1 None 31.2 A 200 White 10 61 Comp. Ex. Yes
Paint 1 None 31.2 A 200 White 10 62 Comp. Ex. Yes Paint 1 None 32.9
A 140 White 10 63 Comp. Ex. Yes Paint 1 None 31.4 A 200 White 10 64
Comp. Ex. Yes Paint 1 None 31.3 A 200 White 10 65 Comp. Ex. Yes
Paint 1 None 32.5 A 80 White 10 66 Comp. Ex. Yes Paint 1 None 31.8
A 140 White 10 67 Comp. Ex. Yes Paint 1 None 31 A 80 White 10 68
Ex. Yes Paint 3 None 35.1 A 120 White 10 69 Ex. Yes Paint 3 A 33.8
A 120 White 10 70 Ex. Yes Paint 2 A 33.8 A 120 White 10 Surface
tension Surface tension Upper difference Lower Upper difference No.
layer (lower-upper) No. Primer layer layer (lower-upper) 34 Ex.
Paint 2 A 31.1 A 120 Clear 15 2.7 metallic 35 Ex. Paint 2 A 31.1 A
120 Clear 10 2.7 metallic 36 Ex. Paint 2 A 31.1 A 120 Clear 10 2.7
metallic 37 Ex. Paint 2 B 30.8 A 120 Red 15 3 38 Ex. Paint 1 A 31.8
A 120 Red 15 2 39 Ex. Paint 2 A 31.1 A 120 Red 15 2 40 Ex. Paint 1
A 31.1 A 740 Red 15 2.4 41 Ex. Paint 1 A 31.4 A 180 Red 15 2.1 42
Ex. Paint 1 A 31.1 A 642 Red 15 2 43 Ex. Paint 1 A 31.4 A 297 Red
15 1.7 44 Ex. Paint 1 A 31.4 A 184 Red 15 1.7 45 Ex. Paint 1 A 31.4
A 137 Red 15 1.7 46 Ex. Paint 1 A 31.8 A 120 Red 15 1.7 47 Ex.
Paint 1 A 32.1 A 180 Red 15 1.4 48 Ex. Paint 1 A 31.8 A 106 Red 15
1.3 49 Ex. Paint 1 A 31.9 A 84 Red 15 1.2 50 Comp. Ex. Paint 1 A 32
A 59 Red 15 1.1 51 Comp. Ex. Paint 1 A 32 A 37 Red 15 1.1 52 Comp.
Ex. Paint 1 A 32.1 A 180 Red 15 1 53 Comp. Ex. Paint 1 A 32.1 A 180
Red 15 1 54 Comp. Ex. Paint 1 A 32.1 A 180 Red 10 1 55 Comp. Ex.
Paint 1 A 32.1 A 180 Red 15 1 56 Comp. Ex. Paint 1 A 32.1 A 180 Red
10 1 57 Comp. Ex. Paint 1 A 32.1 A 180 Red 10 1 58 Comp. Ex. Paint
1 A 30.4 A 200 Red 15 1 59 Comp. Ex. Paint 1 A 30.4 A 200 Red 15
0.9 60 Comp. Ex. Paint 1 A 30.4 A 200 Red 15 0.8 61 Comp. Ex. Paint
1 A 30.4 A 200 Red 15 0.8 62 Comp. Ex. Paint 1 A 32.1 A 180 Red 15
0.8 63 Comp. Ex. Paint 1 A 30.7 A 200 Red 15 0.7 64 Comp. Ex. Paint
1 A 30.7 A 200 Red 15 0.6 65 Comp. Ex. Paint 1 A 32.1 A 180 Red 15
0.4 66 Comp. Ex. Paint 1 A 32.1 A 180 Red 15 -0.3 67 Comp. Ex.
Paint 1 A 32.1 A 180 Red 15 -1.1 68 Ex. Paint 2 A 30.1 A 120 Red 15
5 69 Ex. Paint 2 A 30.1 A 120 Red 15 3.7 70 Ex. Paint 2 A 31.1 A
120 Red 15 2.7 Lower layer Surface tension Average Upper Average
Viscosity Color thickness layer No. Primer Paint Leveler value
(mN/m) Fluctuation (sec) (pigment) (.mu.m) Paint 71 Comp. Ex. Yes
Paint 2 C 33.5 B 120 White 10 Paint 2 72 Ex. Yes Paint 2 B 33 A 120
White 10 Paint 2 73 Ex. Yes Paint 2 B 33 A 120 White 10 Paint 2 74
Ex. Yes Paint 2 A 33.8 A 120 White 10 Paint 2 75 Ex. Yes Paint 2 A
33.1 A 120 White 10 Paint 2 76 Ex. Yes Paint 2 A 31.5 A 120 White
10 Paint 2 77 Ex. Yes Paint 2 A 31.4 A 120 White 10 Paint 2 78 Ex.
Yes Paint 2 A 31.4 A 120 White 10 Paint 2 79 Comp. Ex. Yes Paint 1
A 31 A 200 White 10 Paint 1 80 Comp. Ex. Yes Paint 2 A 31.4 A 120
White 10 Paint 2 81 Comp. Ex. Yes Paint 1 None 31.2 A 200 White 10
Paint 1 82 Comp. Ex. Yes Paint 1 None 33.1 A 266 White 10 Paint 1
83 Comp. Ex. Yes Paint 1 None 31.2 A 200 White 10 Paint 1 84 Comp.
Ex. Yes Paint 1 None 31.2 A 200 White 10 Paint 1 85 Comp. Ex. Yes
Paint 1 None 31.2 A 200 White 10 Paint 1 86 Comp. Ex. Yes Paint 1
None 31.4 A 200 White 10 Paint 1 87 Comp. Ex. Yes Paint 1 None 31.4
A 200 White 10 Paint 1 88 Comp. Ex. Yes Paint 1 A 31 A 200 White 10
Paint 1 89 Comp. Ex. Yes Paint 2 A 31.5 A 120 White 10 Paint 2 90
Comp. Ex. Yes Paint 2 A 31.5 A 120 White 10 Paint 2 91 Comp. Ex.
Yes Paint 2 A 31.4 A 120 White 10 Paint 2 Upper layer Surface
tension Average Surface tension Average Viscosity Color thickness
difference No. Leveler value (mN/m) Fluctuation (sec) (pigment)
(.mu.m) (upper-lower) 71 Comp. Ex. A 31.1 A 120 Red 15 2.4 72 Ex. B
30.8 A 120 Red 15 2.2 73 Ex. A 31.1 A 120 Red 15 1.9 74 Ex. B 30.8
A 120 Red 15 3 75 Ex. A 31.1 A 120 Red 15 2 76 Ex. A 31.1 A 120 Red
15 0.4 77 Ex. A 31.1 A 120 Red 15 0.3 78 Ex. A 31.1 A 120 Red 15
0.3 79 Comp. Ex. A 30.8 A 200 Red 15 0.2 80 Comp. Ex. A 31.3 A 120
Red 15 0.1 81 Comp. Ex. None 31 A 200 Red 15 0.2 82 Comp. Ex. None
32.6 A 180 Red 15 0.5 83 Comp. Ex. None 30.9 A 200 Red 15 0.3 84
Comp. Ex. None 31 A 200 Red 15 0.2 85 Comp. Ex. None 30.9 A 200 Red
15 0.3 86 Comp. Ex. None 31 A 200 Red 15 0.4 87 Comp. Ex. None 30.9
A 200 Red 15 0.5 88 Comp. Ex. None 31 A 200 Red 15 0 89 Comp. Ex.
None 33.5 A 120 Red 15 -2 90 Comp. Ex. None 33.5 A 120 Red 15 -2 91
Comp. Ex. None 33.5 A 120 Red 15 -2.1
TABLE-US-00003 TABLE 2 Ra of Interface waviness % Adhesion between
Mottled-like interface (with respect to upper layers Work heat No.
tone score (.mu.m) layer thickness) (coin scratch) adhesion 1 Ex. 5
0.49 7 Good Good 2 Ex. 5 0.42 3 Good Good 3 Ex. 5 0.57 5 Good Good
4 Ex. 5 0.51 3 Good Good 5 Ex. 5 0.58 5 Good Good 6 Ex. 5 0.70 7
Good Good 7 Comp. Ex. 1 0.52 100 Good Good 8 Comp. Ex. 3 0.42 55
Good Good 9 Comp. Ex. 3 0.57 60 Good Good 10 Comp. Ex. 2 0.51 80
Good Good 11 Comp. Ex. 2 0.58 100 Good Good 12 Comp. Ex. 1 0.70 100
Good Good 13 Ex. 5 0.64 7 Good Good 14 Ex. 5 0.43 3 Good Good 15
Ex. 5 0.35 5 Good Good 16 Ex. 5 0.69 3 Good Good 17 Ex. 5 0.54 5
Good Good 18 Ex. 5 0.56 5 Good Good 19 Ex. 5 0.30 10 Good Good 20
Ex. 5 0.25 30 Good Good 21 Ex. 5 0.12 40 Good Good 22 Ex. 5 0.43 7
Good Good 23 Ex. 5 0.63 3 Good Good 24 Ex. 5 0.56 5 Good Good 25
Ex. 5 0.47 3 Good Good 26 Ex. 5 0.47 5 Good Good 27 Ex. 5 0.67 5
Good Good 28 Ex. 5 0.35 10 Good Good 29 Ex. 5 0.21 35 Good Good 30
Ex. 5 0.15 45 Good Good 31 Ex. 5 0.58 7 Good Good 32 Ex. 5 0.49 3
Good Good 33 Ex. 5 0.60 5 Good Good 34 Ex. 5 0.50 3 Good Good 35
Ex. 5 0.30 5 Good Good 36 Ex. 5 0.39 5 Good Good 37 Ex. 5 0.37 7
Good Good 38 Ex. 5 0.38 10 Good Good 39 Ex. 5 0.34 7 Good Good 40
Ex. 5 0.60 7 Good Good 41 Ex. 5 0.66 7 Good Good 42 Ex. 5 0.61 10
Good Good 43 Ex. 5 0.57 7 Good Good 44 Ex. 5 0.57 7 Good Good 45
Ex. 5 0.52 7 Good Good 46 Ex. 5 0.51 0 Good Good 47 Ex. 5 0.69 0
Good Good 48 Ex. 5 0.65 7 Good Good 49 Ex. 5 0.37 7 Good Good 50
Comp. Ex. 4 0.61 65 Good Good 51 Comp. Ex. 2 0.35 100 Good Good 52
Comp. Ex. 4 0.52 55 Good Good 53 Comp. Ex. 4 0.67 50 Good Good 54
Comp. Ex. 4 0.37 55 Good Good 55 Comp. Ex. 4 0.41 60 Good Good 56
Comp. Ex. 4 0.49 65 Good Good 57 Comp. Ex. 4 0.51 100 Good Good 58
Comp. Ex. 4 0.70 50 Good Good 59 Comp. Ex. 4 0.58 55 Good Good 60
Comp. Ex. 4 0.30 50 Good Good 61 Comp. Ex. 4 0.36 55 Good Good 62
Comp. Ex. 4 0.43 55 Good Good 63 Comp. Ex. 4 0.51 70 Good Good 64
Comp. Ex. 4 0.45 60 Good Good 65 Comp. Ex. 3 0.69 90 Good Good 66
Comp. Ex. 2 0.64 100 Good Good 67 Comp. Ex. 1 0.33 100 Good Good 68
Ex. 5 0.28 3 Poor Poor 69 Ex. 5 0.29 3 Poor Poor 70 Ex. 5 0.48 3
Good Good 71 Comp. Ex. 2 0.55 100 Good Good 72 Ex. 5 0.48 3 Good
Good 73 Ex. 5 0.47 3 Good Good 74 Ex. 5 0.64 3 Good Good 75 Ex. 5
0.67 3 Good Good 76 Ex. 5 0.66 3 Good Good 77 Ex. 5 0.41 3 Good
Good 78 Ex. 5 0.54 3 Good Good 79 Comp. Ex. 4 0.34 60 Good Good 80
Comp. Ex. 4 0.52 65 Good Good 81 Comp. Ex. 4 0.55 65 Good Good 82
Comp. Ex. 2 0.65 100 Good Good 83 Comp. Ex. 3 0.69 85 Good Good 84
Comp. Ex. 2 0.66 100 Good Good 85 Comp. Ex. 2 0.30 100 Good Good 86
Comp. Ex. 2 0.62 100 Good Good 87 Comp. Ex. 2 0.63 100 Good Good 88
Comp. Ex. 2 0.37 100 Good Good 89 Comp. Ex. 1 0.62 100 Good Good 90
Comp. Ex. 1 0.68 100 Good Good 91 Comp. Ex. 1 0.62 100 Good
Good
Example 2
A similar method as with Example 1 was used to simultaneously coat
three layers on a steel strip. The paints, pigments, and levelers
used were as described in Example 1. Table 3 and Table 4 show the
results of the experiments.
TABLE-US-00004 TABLE 3 Layer 1 Layer 2 Surface tension Average
Surface tension Average Viscosity Color thickness Average No.
Primer Paint Leveler value (mN/m) Fluctuation (sec) (pigment)
(.mu.m) Paint Leveler value (mN/m) 1 Ex. Yes Paint 1 A 32.1 A 180
Red 5 Paint 2 A 31.5 2 Ex. Yes Paint 2 None 33.5 A 120 Red 5 Paint
2 A 31.5 3 Comp. Ex. Yes Paint 1 None 32.6 A 180 Red 5 Paint 2 A
31.5 Layer 2 Average Surface tension Ra of Interface waviness %
Surface tension Viscosity Color thickness difference interface
(with respect to upper No. Fluctuation (sec) (pigment) (.mu.m)
(layer 1-layer 2) (.mu.m) layer thickness) 1 Ex. A 120 White 10 0.6
0.45 3 2 Ex. A 120 White 10 2.0 0.38 3 3 Comp. Ex. A 120 White 10
1.1 0.33 80
TABLE-US-00005 TABLE 4 Layer 3 Surface tension Average Surface
tension Average Viscosity Color thickness difference No. Paint
Leveler value (mN/m) Fluctuation (sec) (pigment) (.mu.m) (layer
2-layer 3) 1 Ex. Paint 2 A 31.1 A 120 Red 15 0.4 2 Ex. Paint 2 A
31.1 A 120 Red 15 0.4 3 Comp. Ex. Paint 2 A 31.1 A 120 Red 15 0.4
Ra of Interface waviness % interface (with respect to upper
Mottled-like Interlayer adhesion No. (.mu.m) layer thickness) tone
score (coin scratch) 1 Ex. 0.66 3 5 Good 2 Ex. 0.66 3 5 Good 3
Comp. Ex. 0.66 55 4 Good
Example 3
Using the ratio as shown in Table 5, paints for the lower coat
layer and upper clear layer were prepared. Specifically, the
individual polyester resins shown in Example 3A to 3E of Table 5
were dissolved in an organic solvent cyclohexanone/Sorbesso (name
of product of Exxon Chemical) 150. Next, in accordance with need, a
melamine resin (methylated melamine (product name: Cymel 303, made
by Mitsui Cytec)) or an isocyanate curing agent (Desmodur BL3175,
made by Mitsui Cyanamide) was added to each of these paints, then
in accordance with need a catalyst (product name: Catalyst 6000,
made by Mitsui Cytec) or TK1 (made by Takeda Chemical) was added,
then the results were stirred to obtain the paint used.
To adjust the surface tension, a leveler BYK141 (made by
BYK-Chemie) was added to the lower layer paint in an amount of 0.1
wt %, while an acryl-based leveler (made by Nippon Paint) was added
to the upper layer paint in an amount of 0.3 wt %. The surface
tension was measured by a Dynometer (made by BYK-Chemie GmbH).
Galvanized steel sheet plated on both sides with an amount of
deposition per side of 60 g/m.sup.2 and having a thickness of 0.8
mm was immersed in a degreasing treatment agent (made by Nippon
Parkerizing) to degrease it, was washed with water, then was dried.
Next, the degreased galvanized steel sheet was coated by a roll
coater with a chromate-free pretreatment agent (E300N, made by
Nippon Parkerizing) and dried by hot air under conditions giving a
peak metal temperature of 60.degree. C.
After the nonchromate pretreatment, a slide hopper type curtain
coating system was used to form the paint films shown in Table 5 on
a galvanized steel sheet after the chromate-free pretreatment by
simultaneous coating. These layers were simultaneously baked (peak
metal temperature (PMT) 235.degree. C., 1 minute) to form on the
galvanized steel sheet a lower coat layer (thickness of 15 .mu.m)
and an upper clear layer (thickness of 10 .mu.m or 1 .mu.m).
Example 4
A resin layer for forming the lower coat layer was formed on a
galvanized steel sheet as used in Example 3 after chromate-free
pretreatment thereof using a roll coater, and was then dried by hot
air at a PMT of 215.degree. C. for 45 seconds. Next, a resin layer
for the upper coat layer was formed using a roll coater, and was
then dried at a PMT of 235.degree. C. for 1 minute.
The coated metal sheets obtained in Examples 3 and 4 were evaluated
for characteristics.
The Ra of the interface was evaluated by the above-mentioned
method.
A super UV tester (Iwasaki Electric) was used for an accelerated
test of the weather resistance of the coat layers. A cycle of
standing under UV irradiation for 24 hours and in an atmosphere of
50.degree. C. and 95% relative humidity for 24 hours was repeated
five times and then the appearance evaluated.
Further, the coats were cut by a cutter knife into a grid of 1 mm
squares, then a commercially available adhesive tape
(Cellotape.RTM.) was used to conduct a peeling test to evaluate the
adhesion of the coat layers. Coat layers with no remarkable
abnormalities in appearance were evaluated as being "good" in
adhesion, while coat layers with interlayer peeling were evaluated
as being "poor" in adhesion.
TABLE-US-00006 TABLE 5 Lower layer Upper layer Main Curing Surface
tension Main Curing ingredient agent Catalyst Thickness (mN/m)
ingredient agent Ex. 3A Polyester Melamine *2 15 .mu.m 33.5
Polyester Melamine resin (brown) resin *1 clear resin resin *1 Ex.
3B Polyester None None 15 .mu.m 33.2 Polyester Melamine resin
(brown) clear resin resin *1 Ex. 3C Polyester Melamine *2 15 .mu.m
33.5 Polyester None resin (brown) resin *1 clear resin Ex. 3D
Polyester Melamine *2 15 .mu.m 33.5 Polyester Isocyanate resin
(brown) resin *1 clear resin resin *3 Ex. 3E Polyester Melamine *2
15 .mu.m 33.5 Polyester Melamine resin (brown) resin *1 clear resin
resin *1 Ex. 4 Polyester Melamine *2 15 .mu.m 33.5 Polyester
Melamine (comp. Ex.) resin (brown) resin *1 clear resin resin *1
Upper layer Results of weather Surface tension Method of Ra of
resistance Catalyst Thickness (mN/m) production interface
acceleration test Ex. 3A *2 10 .mu.m 32.0 Multilayer 0.6 .mu.m Good
simul. coating Ex. 3B *2 10 .mu.m 32.0 Multilayer 0.3 .mu.m Good
simul. coating Ex. 3C None 10 .mu.m 31.8 Multilayer 0.7 .mu.m Good
simul. coating Ex. 3D *4 10 .mu.m 32.1 Multilayer 0.5 .mu.m Good
simul. coating Ex. 3E *2 1 .mu.m 32.0 Multilayer 0.3 .mu.m Good
simul. coating Ex. 4 *2 10 .mu.m 32.0 2C2B 0.1 .mu.m Poor
(interlayer (comp. Ex.) peeling) *1 Methylated melamine (Cymel 303,
Mitsui Cyanamide polyester resin/melamine resin = 70/30 blend) *2
Catalyst 6000 (Mitsui Cyanamide) 1.5 phr blended with respect to
resin solid content *3 Isocyanate curing agent (Desmodur BL3175,
Sumitomo Bayer Urethane (blended to give NCO/OH = 1.0) *4 TK1
(Takeda Chemical), 0.3 phr blended with respect to resin solid
content
As clear from Table 5 showing the experimental findings, the
precoated metal sheets of Examples 3A to 3E of the present
invention did not exhibit any interlayer peeling between the lower
coat layer and upper clear layer or chalking even after the
accelerated test for weather resistance.
On the other hand, the precoated metal sheet of Example 4
(comparative example) exhibited interlayer peeling between the
lower coat layer and upper clear layer after the accelerated test
for weather resistance.
Example 5
In this example, a precoated metal sheet provided with both
workability and corrosion resistance will be explained.
As the metal sheet, a galvanized steel sheet (thickness of 0.6 mm,
width of 200 mm) subjected to a nonchromate treatment (E300N made
by Nippon Parkerizing) as the undercoat treatment was used.
As the lower layer paint, a paint obtained from Nippon Paint, which
was prepared from a polyester (molecular weight of 12000) in an
amount of 100 parts by mass, a curing agent (melamine resin in an
amount of 20 parts by mass, an epoxy resin in an amount of 20 parts
by mass, or a urethane resin in an amount of 10 parts by mass), and
a solvent (mixed solvent of 1:1 mass ratio of Anone
(cyclohexanone)/S150 (Sorbesso 150), however, with the lower layer
paint of no. 7, 40% of the mixed solvent replaced with EEP
(ethyl-3-ethoxypropionate)) in an amount of 170 parts by mass, was
used (the amounts of the ingredients other than the solvent being
expressed as solid content). The rust preventing pigment shown in
Table 6 was added to the lower layer paint. The amount was made an
amount giving the average amount of rust preventing pigment shown
in Table 6 in the lower layer formed by baking. Further, a leveler
(BYK141 made by BYX-Chemie) was added to the lower layer paint in
an amount of 0.1 wt %.
As the upper layer paint, a paint obtained from Nippon Paint, which
was prepared from a polyester (molecular weight of 15000) in an
amount of 100 parts by mass, a melamine resin curing agent in an
amount of 20 parts by mass, a titanium white coloring pigment in an
amount of 80 parts by mass, and a mixed solvent of a 1:1 weight
ratio of Anone/S150 in an amount of 200 parts by mass, was used.
The upper layer paint had added to it an acryl-based leveler (made
by Nippon Paint) in an amount of 0.3 wt %.
The lower layer and upper layer paints were simultaneously coated
on steel sheet by a slide curtain coater and simultaneously baked
at the peak metal temperature (PMT) shown in Table 6 to prepare
samples (in the case of simultaneous baking, in Table 6, PMT shown
only in the data column of the upper layer). The upper layer formed
by the simultaneous baking included the rust preventing pigment
dispersed from the lower layer in the average amount shown in Table
6, but no rust preventing pigment was detected at the surface
layer. As a comparative example, 2-coat 2-bake (2C2B) samples
(paints of the lower layer and upper layer were separately coated
by curtain coating and baked) were also prepared.
The surface tensions of the paints and the centerline average
roughness Ra of the interface between the lower coat layer and
upper coat layer were measured by the methods explained above.
Each prepared sample was evaluated for workability and corrosion
resistance as follows.
The workability was evaluated by conducting a 180.degree. bending
test at 20.degree. C. and finding the limit of the number T where
no cracks or peeling is observed when a coat of a bent part is
observed by a magnification of .times.30 (the number T
corresponding to the number of sheets of the same thickness
sandwiched between parts bent 180.degree., for example, 1 T when
sandwiching one sheet and 2 T when sandwiching two sheets). For
example, when cracks are observed when bending by 2 T and cracks
are not observed when bending by 3 T, the score was 3 T.
The corrosion resistance was evaluated by running a saltwater spray
test based on JIS Z 2371. The lower half of a sample was cut with a
cross and the width of the coat peeled off by taping after 240
hours was measured.
The results of the evaluation are shown in Table 7.
TABLE-US-00007 TABLE 6 Characteristics of Coats Upper layer Lower
layer Rust Rust preventing Thick- Surface preventing Thick- Surface
Ra of Metal pigment ness tension PMT pigment ness tension PMT
interface No. sheet Resin Type Am't (%) (.mu.m) (mN/m) (.degree.
C.) Resin am't (%) (.mu.m) (mN/m) (.degree. C.) (.mu.m) Remarks 1
GI Polyester/ SC 25 8 33.2 215 Polyester/ 0 15 31.4 230 <0.1
Comp. Ex. melamine melamine 2C2B 2 GI Polyester/ SC 25 8 33.2 --
Polyester/ 5 15 31.4 230 0.3 Invention melamine melamine 3 GI
Polyester/ SC 35 8 33.2 215 Polyester/ 0 15 31.4 230 <0.1 Comp.
Ex. melamine melamine 2C2B 4 GI Polyester/ SC 35 8 33.2 --
Polyester/ 8 15 31.4 230 0.3 Invention melamine melamine 5 GI
Polyester/ SC 25 8 33.3 -- Polyester/ 5 15 31.4 230 0.6 Invention
epoxy melamine 6 GI Polyester/ SC 25 8 33.5 -- Polyester/ 5 15 31.4
230 0.7 Invention urethane melamine 7 GI Polyester/ SC 25 8 33.2 --
Polyester/ 5 15 31.4 230 0.9 Comp. Ex. melamine melamine 8 GI
Polyester/ CS 35 8 33.2 215 Polyester/ 0 15 31.4 230 <0.1 Comp.
Ex. melamine melamine 2C2B 9 GI Polyester/ CS 35 8 33.2 --
Polyester/ 8 15 31.4 230 0.3 Invention melamine melamine 10 GI
Polyester/ SC 52 8 33.2 -- Polyester/ 12 15 31.4 230 0.4 Invention
melamine melamine 11 GI Polyester/ CS 52 8 33.2 -- Polyester/ 12 15
31.4 230 0.5 Invention melamine melamine GI: galvanized steel
sheet; SC: strontium chromate; CS: calcium silicate
TABLE-US-00008 TABLE 7 Coat Performance Corrosion resistance No.
Workability (mm) Remarks 1 3T 1-3 Comp. Ex. 2C2B 2 1T 0.5-1
Invention 3 4T 1-2 Comp. Ex. 2C2B 4 1T 0.2-0.8 Invention 5 1T 1-2
Invention 6 1T 1-2 Invention 7 3T 2-3 Comp. Ex. 8 5T 1.5-2.5 Comp.
Ex. 2C2B 9 1T 0.5-1 Invention 10 1T 0.1-0.5 Invention 11 1T 0.1-0.8
Invention
Nos. 1, 3, 7, and 8 are comparative examples. Nos. 1 3, and 8 of
2-coat 2-bake (2C2B) having centerline average roughnesses Ra of
coat layer interfaces of less than 0.1 were insufficient in
workability, while no. 8 was also insufficient in corrosion
resistance. In no. 7 with an Ra of the coat layer interface of a
large 0.9, the workability and the corrosion resistance were
inferior, the distribution of the rust preventing pigment was
uneven, some of the rust preventing pigment appeared at the surface
layer, and the appearance was poor.
* * * * *